Back when I worked at JPL in the late 1970s I was the most junior engineer, so my time slot on the computer (an SDS 930) was from 2AM to 4AM Monday, Wednesday, and Friday.

Over time I got to know the guard who patrolled our building and we would hang out and chat when he came by and found me there with the machine room door open.

After a while I was ordered to keep the computer room door closed, since keeping it open was unbalancing the A/C.

Nonetheless he somehow managed to come in and find me quite regularly.

One night I asked him how he knew when I was working.  He told me that my program, when it was running, made a characteristic sound on his walkie-talkie.