I've been reading this with interest. I found a small shih tzu wandering outside my apartment corridor (i live on the ground level though so he could have come from anywhere) I tried to find owners but my notices got no calls. Asking around the neighbourhood also didn't help. He is very cute and well behaved except for some toilet training issues, and when i cute his hair short and washed him, found some rashes.
we though he was old coz he was much less lively than my 7 yr old dog, but a visit to the vet told us he was under 2 yrs old.

I was also told he had no microchip. Microchipping is not that expensive and i feel if people cant be bothered to register their animals they should not have them. After a few toilet accidents we came to the conclusion that he was likely to have been abandoned. Coz like OP said, the dog looked very lost walking the corridor and followed me home with no hesitation. He didnt exhibit any 'runner' or 'escape artist' behaviour. I've had escape artists before.

I kept him for a week. We can't have two dogs. yesterday the new owners came to get him. I asked them to please microchip and register him and my friend said she would. We kinda miss his presence, my dog seems to miss him the most. She looked for him for awhile and gave us some bewildered looks.

I wish I could do this for some of my students.