I was going to tell you to not syat anything and watch the disappearing ones and the next time one disappears without reason...confront him, but after reading further it sounds like you need to just confront and do what you must.
I have been counting viagra for years...when he travels I make sure I know how many are there just before he leaves and check his pill case in his bag, etc and just when he comes back.
Any time he has one left in his case from a trip we took together,he returns it to medacine chest.
The last business trip he took, he had one pill in case that was there from a previous trip we took together...I waited for him to put it back in the bottle but he did not, so Igot into his briefcase the morning he leftand removed it...he probably noticed, but did not mention and I did not put it back in bottle in medicine chest, so he may think he lost it...i don't know????
Today he left for a business trip with one in his case...he did not remove it and this time I did not remove it. i know how many are in bottle and that the one is in his pill case and when he returns, it should still be there...if it is not, we will behaving a serious talk and it will be over because ther IS NO GOOD EXPLAINATION for them to go missing...end of story.
I would not bother trying to track where he bought them ...most perople buy online now I think ...it is bad enough not to trust them not to cheat, but what is even worse is not being able to trust that they will tell you if they do.