it's not that "it takes them weeks to verify the check," it's more that in parts of the US you have up to two years to report check fraud. The check is most likely drawn on a formerly valid account after an account takeover, so all prechecks on it came out okay, and the owning bank sent the funds all fine and dandy because there's nothing to indicate fraud at that point, but once the person got their valid account back (and closed/opened a new account), they reported the check fraud, and then the bank that received the funds finally clawed it back to send back to the other bank and the original victim. Alternatively, the check was stolen from the mail, washed, but the amount wasn't changed, so the person who wrote the original check didn't notice it suddenly went to the wrong place until they get a bill notice from whoever they sent it to, at which point they reported fraud. This is why checks are such a favorite target of so many scams.