workplace disaplaine

Trying to find a way to deal with a issue at work. In Maine if helps.

In my department there a door to the outside that is alarmed. Not linked to fire alarm or such just one that if opened just loud buzz. So that managers know when its open. We sometimes use the door but are required to let a manager know they come disarm it and then rearm it after we are done. On Nov 25th someone messed with the alarm to make it so it could be opened with out setting it off. While there are no video cam to have 100% proof of who did it. Most think they know who did it. We have a employee who has been nailed a few times just opening it up and letting it buzz or putting tap over the speaker to try and damping the sound. He hate it and thinks it stupid.

On Nov. 27th we learned that the district manage has said unless who ever did it comes out and admits it everyone in the department is getting a final writing warning. not enough to get fired but enough so one more issue and your gone.

Any suggestion on how to deal with this. We know the person we all suspect of doing it don't care if everyone gets in trouble. hes has a full time job with the city and just works 2 days a week with us.