The bill acceptor, did just that.

Many bill readers in slot machines read a "signature" from the magnetic ink used in USA bills but apparently the bill counters in the count room use more sophisticated image recognition.

When the bills were rejected in the count room, the security cams revealed the faces (but not the identities) of the passers.

What identified them was their SLOT CLUB CARD, which they had inserted in the machine prior to passing the bills.