footsteps are everywhere.
This is what threw me off from the beginning. Do you mean footprints, or do you actually hear footsteps after you investigate the shed?
I don't think it matters.
Considering the question:
how did they get into the shed quickly and quitely?
It sounds like it's the only thing that really needs solving, and they could have broken into the shed quietly in any number of ways. Whether it's a bobby pin or a lock-picking kit or a bolt cutter, you aren't going to hear any of it if you're asleep.
I think either the question is too vague and needs rephrasing, or the riddle is very simple and the intended answer is that the robbers used the bobby pin to break into the shed.