We've had a couple of customers who have sent an inordinate amount of money (from our perspective) to Yemen all via check. The funding source was most often cash. We ended up closing the account because the explanation for the activity didn't make sense.
We also had several that sent checks in much smaller amounts, less often and funded by their normal income (payroll). Those appeared to be support of family and we didn't close them.
Of all the things I've lost, I miss my mind the most.