I have a 1.5 year old male chihuahua. He was fixed some time ago. His schedule has not changed. He uses both pee pads and goes outside. For the last couple of months he will randomly poop next to me, and my husband, or our kids like it's no big deal. I am not sure what messages is sending, but it is getting very frustrating.

What kind of exercise is this dog getting?
Even though they are small, they still require regular exercise and mental stimulation. I would suggest upping his walks to see if that stops it. Most dogs when let out themselves to 'do their business' don't exercise themselves very well.
This could be as simple as him just needing a bit more stimulation - some dogs react to it in this way. Try taking him out more and see if that works.

I believe a 1.5 year old dog can be confused by a pee pad and I see no reason for him to need one. A pee pad sends a mixed message to a dog. The fact that he uses one tells me that you might not be taking him out often enough. When do you feed him and when is he walked or turned out?

I agree that the pee pads and exercise routine can be the main factor. A dog allowed to pee and poop on one soft surface and in the home sees no reason to not be able to pee and poop on any soft surface in the home. If you MUST have a pee pad, try the potty patches, It's plots of grass that you can keep in the home and it acts as a more natural environment, creating less "accidents". I would strongly suggest to nix any and all allowance of bathroom breaks in the house.