A few months back I got my hands on a Monster Mill and was assembling it when I dropped a bolt near an open sack of 2-Row. I rooted around in the top layer of bag for a few minutes but was convinced it hadn't landed there of all places. But that bolt never turned up.
Cut to a few months/batches later:
I happened to be using a pint glass to weigh out grain and there was *clink* in the glass. That reminded me about the possibility of a big old bolt hanging out in the grain bag and I found it...
If that was plastic scoop then I wouldn't have noticed the metal until it was busy un-knurling my rollers.
With any sort of dense metal piece it sinks right down through the grain and it seems to dive through it faster than you can check when inspecting for any such debris.