I was an ME in a hotel after I retired. The cops said the security guard was checking on an open room door alert which I wondered about. This guy said it was about a fire door that wasn’t latched which makes more sense.
I wonder if when the shooter put the metal plate and screws into the stairway door if that messed up the sensor showing the door was open. Although I wouldn't think it would. Unless he accidental drilled into the wiring and broke the circuit. (Murphy's Law and all).