I think small-time pilfering of £5-£100 of things like PTA accounts, small time charities, sports clubs, community associations and religious groups is ubiquitous.
I think a lot of people pay themselves "expenses" off the book, justifying grabbing a fiver out of the cash tin for the stamp they bought, and "meaning to put the change in later" and its one of those things that is often turned a blind eye to, especially where its obvious the person is struggling.
Often there are deeper issues like addictions, debt, family problems that lead that to escalate to the point where it becomes it gets stamped down upon. But sometimes that doesnt happen either because the person started getting sneaky, or because eventually other people become complicit either in covering it up, or benefiting from the situation.
I've been involved in three situations where this has happened. In one, someone simply ran off with the cash (hundreds) and was never seen again. In the second (thousands), the treasurer was making up false accounts and forging the co-signatory, who was receiving what ever "expenses" he asked for without question, it all came out when the account was drained and payments started bouncing. And in the third, the person who banked the money had managed to get her bank account linked to the charity account and was making large payments to herself under the guise of a supplier (tens of thousands). as the committee could not be bothered with addressing her belief that she was employed by them, so she just paid herself and they all ignored it.