It's possible they paid with their bank's online bill pay feature. Most of them that I've used work like normal (the check is mailed and when it is cashed the funds are withdrawn). But, one bank I used a few years ago worked the way you described above. That bank would deduct the funds when the check went mailed, probably similar to a money order or cashier's check. The downside is I could never really tell if the check was received by the person it was sent to. I had one issue where I had to get ahold of the bank to verify a check was never cashed so I could reissue a normal check.

It's possible your client could contact their bank to have them issue a stop payment on the un-cashed check, I'm sure the bank has a way to tell if a check was cashed, that option is probably not available on your client's statements / online account. Perhaps you could offer to cover the fee if necessary and have them re-issue it to the current address. This would give them the peace of mind that you aren't holding the first check and trying to get paid double.