Yeah any warning system will just turn into California’s prop ~~86~~ 65, which led every building in the state to add signage reading “this building may or may not contain cancer-causing chemicals.” Which is completely useless.
Knowing that you have facial recognition and eliminating it is much easier.
All it takes is UPS adding facial recognition to driver bodycams or something and suddenly your building has facial recognition in it. Better to just put the warning everywhere and forget about it.
If your place of business is actively adding cameras, and is actively using facial recognition, those are actions that your business has taken. A temporary action of another party should require you to do a disclaimer. Especially if the same action is something they should be disclaiming.
As for facial recognition in common areas, the landlord can put a clause in the lease agreement requiring tenants not to do that.
If disclaimers are required for facial recognition and a UPS driver has a facial recognition bodycam, I assume it'll be the UPS driver that needs the disclaimer, not every place the UPS driver visits.
A chemical can't be seen, and can't be removed.
And even if the hardware is super entwined with the building, making it hard to remove, such as tied in with the HVAC system, there likely will be some toggle switch to turn off facial recognition. There's no toggle switch to remove chemicals.