He stashed it between boxes instead of leaving it in plain sight. That's "hidden" by any conventional definition of the word.
'Stashed' means he intends to come back for it. You know, like CTers and their wash, rinse & repeat schtick to live up to Oswald's motive to be remembered for the next 10,000* years.
*For High school drop-outs: Now 9,942 years.