We've had a mouse living in our worm bin for a few weeks (I've seen it
dancing around on top of the heap when I tip stuff in).

The bin is the open-bottomed "dalek" type. I have one of those oblong
metal box mousetraps with two one-way gates one side & two spring
traps inside, arranged so that a "flag" is visible through the top
when a trap's been sprung. I originally put this down under a nearby
wall of the shed (the shed walls have ventilation gaps of 40 to 50
mm), assuming that the mouse would run that way; after a while, I
moved it under the wall closest to the worm bin.

I now wonder if the mouse is living more or less permanently inside
the bin --- is that possible?

I'm reluctant to put any kind of poison in the worm bin since I use
the output in the garden & (when I'm lucky) grow something edible.

I'm seriously considering setting the trap-box on the top of the
contents of the worm bin. Is that a smart or stupid idea?

Thanks.