Never been in one of those, but check things like the heat sink fins, and any spaces underneath circuit boards where the screw could have gotten lodged...
I bought an old synth as a restoration project years ago, and on initial inspection had found one of the enclosure screws had some loose and lodged itself right between the power supply board and metal enclosure. You can see it peeking out in this pic:
At least it was on the low voltage side of that board, and not shorting against one of the mains connections!
