Agreed, certainly seems like a good time to reach out to your customer to inquire as RockChucker has stated.
I would also be interested in the dates that the non-customer is cashing the additional item at the other FI, since you're aware that they're cashing a total of 10k from your customer in 2 separate checks. Could be a good way to get around the CTR if they're doing some at your FI and some at another, just as possible speculation, especially if they're done on the same business date.
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