Would you bring him back or would you just find him a new home where you know he will be loved and cared for? and if they ask about him say we haven't seen him.
Absolutely not!!! If the cat "flinched" chances are the cat is either NOT being taken care of (found roaming the hallways??? Um...NO!) or is being abused.

Make him go "poof". Tell the FORMER OWNERS that the cat must've gotten out of the building and you haven't seen it.

Am I asking you to lie??? Hell yeah!!! Sometimes you HAVE to in the name of animal rescue. These people OBVIOUSLY don't care what happens to the cat. Please find him a furrrrever home where someone will love and PROTECT him!!!