"Riddle Me This!"

The first thought that comes to mind, as there are few details, is that the dead body was inside "your old stuff" [boxes, maybe?], which you were moving into the storeroom. You weren't looking inside the stuff, probably, but rather just moving it, so you could have moved the body inside without knowing it. Now for the numerous problems: it seems unlikely that you wouldn't look inside the boxes (if they were boxes) at all while moving it, you probably would have noticed the weight of a corpse, and there's no way that I can see that the corpse could have come out of the boxes during the night, which it had to do.

the padlock that "...that can be closed without a key but requires a key to open..."