Sealed crawlspaces do sometimes seem to accumulate stuff in
inexplicable ways. I'd be curious about what sorts of "treasures" you
might have found. When the original floors of a mid-18th-c house were
taken up here in Richmond for repair recently, my wife, Robin Ryder,
excavated beneath the joists, primarily searching for dating material
in builder's trenches, etc. She found large ceramics and other things
which we cannot figure out how they got there.
 
The removal of floorboards to make a hiding place is a widespread
practice (at least in film noir detective stories. We own a 19th-c
house which shortly after buying it we found a loose floorboard in an
upstairs back room (maid's chamber). Under the floorboard there was
stashed a complete set of rosary beads and crucifix.
 
Dan