Here in Adel we always take the cat back into care if it came from us. In fact recently we have re-homed a 6 year old who was adopted from us when he was a kitten, because his meowmy has terminal cancer and wanted to prepare well in advance
We didn't have space for him so we advertised on our website and he's found a new home.
We don't set a time limit on ours and always tell people to come back to us if there's a problem!
On the Tree site - which area is it in?
I would imagine (but might be speculating) that they've contacted the branch and been tld they're full but can go on a waiting list to come into the branch, so the people have started to look at re-homing themselves?