Relearning Object Permanence

My life tends toward routine: same trains, same paths, work, home, same thing for breakfast and lunch. So the signal on the subway platform at Union Square as I'm changing trains on the way home from work catches me by surprise. 

It's not a small thing, this signal, at least as tall as me, and it's chunky and made out of pitted, knobby metal that's been painted black so many times that the once sharp, machined edges of it have started to soften and round, with a bright red light shining on the top, and a currently dark light colored green on the bottom.

But it surprises me, because I get off the train at the same place everyday, and I don't remember it being there yesterday, and yet it's not entirely unfamiliar to me, like I've seen it recently, which makes me wonder if it's only there on certain days, like Brigadoon, or if it's only there when I notice it (both ideas being, of course, entirely insane).