Quote from Wile E Coyote
Yep, whatever indicator that's used will eventually ruffle some feathers, so it'll only work with your leadership's backing.
Where I work (and probably in many other places), in due time, a visitor will likely remove the magnet to take home for the refrigerator magnet collection.
I wish I were totally kidding...
More seriously, staff in general should be notified via report if the person is really that unstable and likely to act out with aggression. In some cases, a rather benign trigger brings out this behavior, like being startled by a staff member who enters the room and speaks to the patient at close range.
Staying out of reach of the patients hands, and never standing within "kick range" are important considerations. I also teach staff to "never allow such a patient get between you and the door", and to never turn your back on such patients (aka, "watch your six")
Many hospitals teach TCI (Therapeutic Crisis Intervention), which includes many ways of remaining safe, as well as maneuvers for 'takedowns' and safely releasing a patient's tight hand grip. Much of the class teaches effective ways to de-escalate a tense situation.