Well there can be many reasons why your till in short, but I like that you at least care about it. Do you always count your drawer? Was this at the Checklane or Office?

Tills can be off for a few reasons:
-Till was not setup with the correct $$ in the morning
-Quick Changed
-Gave too much $$ as change
-Tendered Cash, that should have been another payment
-Did not process check for written amount/did not correct amount at POS/VB
-Just counted the $$ wrong

Personally I wouldn't worry too much, but just be more careful. If anything show concern to the bookkeeper/CSM.....