The dates on the checks should give a pretty big clue as to when it happened.
I know of a church in Dallas where they found a deposit bag above the ceiling tiles in the men’s room. All the cash was gone but the checks were still there. Needless to say, reasonable proceedures were put in place to prevent a recurrance. (Due to demographics, cash is a negligible fraction of contributions).
It always amazes me to learn that some trusted employee, who is also considered part of the family by the owners steals hundreds of thousands over a period of years and the embezzlement is accidentally discovered