Apparently there are parts of the Santa Clara County jail that are very old. This occurred in a dorm-style housing unit, and the key he made opened a stairwell door (the door to the stairwell being inside the housing unit, which is how he was able to repeatedly test without scrutiny). The door was alarmed and went off when it opened. Most modern jails don't use skeleton-style keys, and the doors are opened and closed electronically, but the part of the jail he was in was decades old.