Note: nothing below is meant to offend. It's simply insight pertaining to the subject matter of this thread.
First, there's a misconception here about holsters. Not all holsters allow someone to remove the gun other than a trained individual. Some officers don't use them. Prepared officers do. There are holsters you could bolt to the ceiling, place the pistol in it, and then hang from the handle of the weapon without it coming out. End of that topic.
Second, contact the departments likely to visit your facility and have something in writing with command. We'll cooperate.
Third, never demand an officer surrender his weapon without the above. It's an insult and often unlikely to happen without a superior officer instructing him to.
We're used to securing them for correctional facilities so going into a psych unit with a gun shouldn't be an issue. Try having a sign though so an officer knows ahead of time. If an out of town officer came to pick up someone then him or her seeing the sign would serve adequate notice. Popping out from behind a desk shouting "you can't bring your gun in here" isn't a pleasant experience for either party involved. I've only been in a true psych unit as a paramedic and not an officer. I have had a security guard at a hospital tell me in uniform
that I couldn't carry my weapon inside. I simply said, "as a matter of fact I can" and shouldered past him. It's part of the law enforcement sub-culture. No one takes our weapon. It's a sensitive and personal topic.
Some correctional facilities even require that officers secure their spare magazines (ammunition) as well as batons and so forth before entering.