In my experience it's not so much that you can forget where they are, but that it becomes such a natural part of your stuff that you're not thinking every second "I have a gun". Of course you still know that, and in an alarm situation your hand instinctively knows where to go to draw. But the part about "I'm going in a place that doesn't allow guns..." doesn't connect in time.
I carry every day now, and I've carried on and off for over twenty years. I've never inadvertantly carried a gun into a restricted area-- but I've come close. A few times I've been just about to go through the door of a tavern to get a beer... and just caught myself and turned around to go put it in the truck. I can see how it can happen. In no case had I forgotten about the weapon. It was the ~restriction~ I wasn't thinking about.
I ~have~ accidently carried a knife on an airplane after they were banned. I've also lost a knife at security that I hadn't found in my pre-flight bag check. I checked the bag, thoroughly I thought, but not thoroughly enough. I've got lots of knives and I usually have two or three on my person, and again, it's not the knife that gets "forgotten" but the "restriction" that doesn't get connected.
IMHO
It’s a tough thing. I hate seeing a brother-in-arms being taken down because of an honest mistake. But carrying a weapon is a responsibility along with a right.