How do blind people know where to find Braille signs?

On many occasions I have seen signs, with the equivalent in Braille underneath the text. Which is great. But what baffles me is this: how do blind people know where to find the Braille text?

I don't have any photo examples right now, but in one example I was on a train and noticed a sign on the window saying "Emergency exit, break glass". Underneath that was the Braille for "Emergency exit" (note, no actual instructions). And I couldn't help but wonder, in order to be read a blind person needs to run a finger across it, but why would they run their finger across a random part of the window?