It would certainly raise my eyebrows and make me question why someone not in uniform is walking around with a handgun in plain sight.
Plainclothes security guards, off duty peace officers, business owners, whoever, may be carrying, but if the gun becomes visible, common sense dictates to me to be on the alert and quickly scan for exits.
The truth is that someone plotting a robbery or other evil will probably not have a handgun in plain sight. Someone carrying openly is not up to no good. I realize that it may make some people concerned though. I usually carry concealed, though some of my friends are big proponents of open carry. I’m sure that carrying openly will invite a lot of scrutiny and probably hassles from uneasy people. But the criminals, they have their guns in their pockets or waistbands if they have one at all.