I have a 16 month old terrier mix, who has gotten herself banished from several rooms in the house, for pooping in them. For example, she used to poop in the baby room any time I was in there. Not because she needed attention, but I am assuming because she smelled the diapers in the diaper genie, and drew the conclusion, "I smell poop here, so it must be ok to poop here" since I could only remove the smell that humans are sensitive to, I was forced to prevent her from ever going into that room with me. Which resolved that problem. She has had an accident or two along the way, which she was not punished for, accidents happen. But this past week she has decided that it would be a good idea to poop in the spare bedroom. We use that room mainly for storage, and I have to keep the door open, so the temperature in the house stays the same all the way through. I am a stay at home mom, so she gets to go out anytime she needs during the day, but last night when I was getting ready for bed, I heard crunching that I should not have heard, I still don't know what she was chewing on in that room, but as soon as she heard me headed her direction, she ran back to her bed, as though she had done something bad. What I found was a nice fresh poo and pee spot, I had taken her out less than an hour before. This isn't the first time she has done this either.

There have not been any major changes lately,
It wasn't a cry for attention, otherwise she would have done in front of me, instead of a place I wouldn't find it right away.
She is in perfect health, and there have been no changes to her diet.
She has canine separation anxiety, which is why she has to be put in her crate any time we leave. We had to use a no-bark collar for a long time with her to get her to keep from disturbing the neighbors while we were away. She has learned the kennel time is quiet time though.

What can I do to figure out why she has suddenly decided to secretly poop in the spare room, when everyone is home? And what can I do to fix it? I want to nip this in the bud early, so we don't completely lose all of our house training.

Many dogs are very sensitive to very slight changes, and all I think I can say are creatures of habit.
New babies, changes in hours you are home, moving furniture even.
They also tend to view unused storage areas as OK to use as bathrooms. Also closets and corners crowded with stuff.
Yours may simply have been not ready to go when you took her out, not a deliberate holding it in until later.
I don't think you can say that a dog who wants attention will go in front of you - that's too much a human child trait, not dog.
In short, don't read too much into this, and don't ascribe human traits.
Put a barrier across the storage room if you won't close the door. One good way to housebreak is to start with a small space and gradually enlarge it, and make it smaller again if there are lapses. Close off as many rooms as you can for a while, even if you use cardboard barriers or closed doors. Clean areas she used thoroughly and put some horrible strong smelling product over the spot. There are dog products, but cheap cologne or aftershave or pine scented ammonia (ugh) work just as well.

If she needs to go she usually gives me the cue that she needs to, rather than running into a different room and going. The only reason I figure dogs do bad or good things for attention, is because it is in everything I have read on the subject. Come to think of it, our couch did get replaced last week. With another identical couch, but it probably smells different. Didn't even dawn on me because it is an identical couch.

I had watched a clip about dogs on Animal Planet or somewhere and learned that smells/odors are most of a dog's world (that's why they are such good drug-sniffer-outers at airports, etc.). The couch exchange sounds like it might be the key to this episode.

16 months old is too old for her to be doing this.

When was your baby born? Was your dog in your home first? Or did the baby come first and then you've gotten the dog.

Babies can cause dogs to do strange things. They can develop behaviors that have never been seen and the can relapse and basically protest their house training.

Since your dog started pottying in the baby's room first, I would tend to say that she is upset over the baby. Even if baby was there first, your dog wants to dominate over the baby. Just because she is now not doing it specifically in the baby's room, doesn't mean that she has forgotten about the baby.

Babies smell different. Dogs don't recognize them as humans... They sound like a wounded animal (the cry), they don't smell the same (diapers) and they don't interact with the dog the way you do. Babies and dogs can be a challenge, especially if you have a dominating personality of a dog... which may be your situation.

I would find a trainer who specializes in behavior modification and explain to them about the defecating in the baby's room. The trainer may have certain exercises for you to do with the baby present.

As for right now, I would never let the dog out of your site. If you cannot watch the dogs every move, put her in her crate. Tie her to you when you are doing house work or if you want to sit and watch TV... DO not let her have her own freedoms.. She needs to earn her freedoms.

Even if your baby was around before you got your dog, your dog is becoming sexually mature and different hormones are becoming active.. even if she is spayed. She is wanting to take over... In dog packs, only Alpha has puppies.. You need to establish that you are in charge and that you are the only one who is going to to reproduce. This can be easily done by making her sit and wait for food, sit and wait for leashes, you walk first through a door with your baby.. your baby always gets fed first with your dog in a down and stay position... Banishing the dog during feeding time isn't natural.. Having your dog see you and your baby eat re-establishes that YOU are alpha and that your "pup" is second to your dog. After everyone is done eating, then your dog is allowed to eat. I would do this for breakfast and dinner.. any mid day feedings with your baby, your dog should be present in a down and stay position.. and give her a treat or something once you are done. Establish the heal position when going for a walk.. You are the leader, you are first for all.. Your dog MUST follow you and your baby.

Anytime a dog acts the way your dog does with a baby.. it is always behavioral. Find a trainer that knows about dogs and babies and go from there.. Limit her freedoms and use your crate.

Good luck

Along with Joypulv's reasons, add that dogs often prefer to 'go', especially BM's, in areas they don't frequent much, if at all. A quick treatment for that may be to feed the animal in those areas. Either they will quit 'going' in there, or stop eating. However, a better long term approach is to get the dog accustomed to being active in all the areas of the house, or securely close those unused (by the dog) areas off, since any unused area is free game to 'go' in, if outdoors isn't available.

I stay home with her all day (stay at home mom) so when she goes to the door, I take her out. Since this started happening I have had to go back to making her sleep in her crate at night, instead of her doggy bed in the room with us. I put a baby gate in the door way to the room she was pooping in, so she promptly decided to poop in our bedroom. :( which is why she gets the crate at night again. I hope that soon as I remind her where it is OK to poop, she will get back to only needing to be put in her crate when we go out.