Ok, so today, I did the same thing with a refund check. Teller I didn't know, she just made me swipe the card for ID purposes, and asked me to sign the back, which I signed in a box. She didn't ask me to sign a receipt or anything. I didn't see her write the account number on the back, either... After I left, I went to the atm, and watched her in the mirror close by. She was standing there looking at the back of that check for about 3 min before she put it away. She didn't pick up a pen that I saw. Seemed like she was rusty with the process. She's the branch manager...
There are possibly big flashing warning signs on the account when I swiped the card. I did change the sig card a while back.