I tend to avoid things like this because I think acknowledging the problem brings more attention to it than quietly ignoring it. We've all played those games where you walk in and the elevator is "out of order", and think "well, how convenient for the map designer!"
My general philosophy is that most people accept the game world and its behavior as-is without too many questions, and good design will seek to minimize the number of times that the player is directly confronted with the unreality of the situation. Right now the game has almost zero interaction with the physical environment, so most players will learn the world as a static world.
Unfortunately, a parking garage presents the problem of how cars get up and down. If I added a ramp (my initial plan) there'd be almost no space for parking, so I chose the elevator route even though it's going to raise questions for some like TennSeven. I'm not sure whether adding an out of order sign would just compound the problem: it would directly confront the player with the unreality of the situation, but this time not just for inquisitive players like TennSeven but for virtually all players. It's hard to measure this, though, because only those who notice a problem will come to the forums. It's hard to say how many never even considered it.
If most people disagree, however, I'm not totally against putting in an out of order sign, or maybe something that indicates the elevator won't move unless it senses the weight of a full car.