That's it. Taco Bell for lunch it is!
Yeah, I'd be willing to bet anything it was their deposit. I agree though, you'd have to be pretty dumb to put your deposit in the food bag.
What some restaurants do, is put the money in a food bag, and another employee drives thru, and picks up the bag to take it to the bank.
They think customers will think it's just a bag of food, instead of thousands in cash.