I don't know too much about Hamsters. I know more about snakes and possums LOL If we get either of those in a roof or somewhere we can't get to them then there are people who will come trap them and relocate them, although the possums have to be relocated back in the area, they can't be taken away.

Hamsters? Heck imagine if you have a male and a female who have absconded? Next thing you'll have dozens. The only thing you can do really is to put out food and water in a cage next to the wall. You also don't want the poor thing dying inside the wall cavity. That would be awful.

I see there is a Hamster society in the UK.
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They have a section called "How to recapture an escaped hamster".
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Food, food and more food. Sounds the way to go.
good luck with it. UGH.