1.) If it is concealed, we don't know they are armed. Legally or otherwise. No harm, no foul.
2.) Nebraska is open carry. So if they aren't waving it around and threatening, then it is legal and no harm, no foul.
We (where I work) are not signed either way. But most know we don't really care one way or the other and just expect folks to behave.
Of course, most of the employees are used to it anyway. Also they have no idea if we might be carrying or not either.
I realize the question covers more than Nebraska, but since that is where we are...