My daughter's 8 yr old schnoodle has pooped in the house once per visit the past 2 weekends she & the dog have been w/me and my 1 yr old teacup schnoodle. The 1st time he did it immediately after I let him back in the house in response to his bark at the door. I've been told he's been doing this at their primary residence for a little while now. I've gotten the impression it's been going on for several months there. He was neutered and trained (both house and behavior) as a pup. He's been living predominantly in my Ex's home for nearly 2 years. I am not aware of any changes in either home environment which might upset him. His health appears to be fine - eating, drinking, relieving himself w/normal frequency and amounts, interacting w/people and other dogs in the same way, doesn't appear to be uncomfortable (NO itching, bottom scooting, restlessness, unusual sleepiness, irritability, etc.) Nor have I noticed any other changes in his behavior. Any ideas why he may be doing this and/or suggestions?

You don't suggest that there is any urgency about their 'accidents', and it has been an ongoing problem for both households, so I would discount any typical medical problems.
Well housebroken dogs usually have some signal they give that they need to go outdoors. Has he been giving any, or are both of you missing them? House dogs also are very resistant to making any messes in the areas where they spend most of their time, like playing areas. Many times, if a dog simply breaks training it is in an area in which they seldom go, like a dining room or extra bedroom. These are areas they are seldom in and feel safe in relieving themselves there and not having to 'put up' with it during their normal activity. It is 'out of their way', but not yours.
An 8 year old dog is also considered late middle-aged, or even 'senior', and is beginning to be more susceptible to some of the things that happen to older pets. His memory may be not as good as it used to be, and not being in your home as often as before may not seem to be familiar enough that he brings his 'housebroken' training with him on visits.
Be more observant of him. Take him out more frequently. Remember, 'a meal in and a meal out' works for dogs, too. If the messes stop, then he is either forgetting, or doesn't care as much about being a good guest, and you have helped him be a good guest again by simply anticipating his needs and being more proactive. You may have to adjust your approach to his needs, as he has apparently adjusted his, possibly without actually knowing he has done so.