A red flag for what? That a player may have lost all their money?
I mean a player that claims rain money (especially large amounts), then withdrawing them without wagering will be arouse the casino's suspicion.
Casino should not allow the user to withdraw that fund
(rain money) in the very first place if that's the case. Or at least include it on their terms. Or require the user to bet at least XX amount, like betting at least 60% of the total rain money collected.
But no, after a user do it
(withdraw request), the account will held on to their custody and ban it.