Just another thought, have you reconciled all the other accounts? I'm not familiar with Sage but use Quickbooks (a similar system) daily and it would be possible for me to enter a bill for a purchase made from petty cash but to accidentaly record that bill being paid from another account, such as the current bank account. The only real way would notice the error is when I went to reconcile the current account.
Also, have you asked around to see if anyone has put some money back in?
As it's petty cash, I can't imagine it is a large amount of money so I wouldn't worry yourself and waste too much time over it. It might be advisable to just go to your boss and tell them them of the discrepancy. As it's the right way round (i.e. no money missing) I'm sure they will tell you just to take it out as you can't easily find the error.