Never saw a Falmer out-of-doors, but their idea of destroying the surface world will be a long time in the making if all they do is wait for spelunkers to come to them. Oh, and apparently there's a couple of guard phrases that could be what was mentioned above:
Whiterun guard: "You know I saw something near http://www.uesp.net/wiki/Skyrim:Shimmermist_Cave to the northeast. Some sort of creature. Small and wicked, never seen its like before."
Windhelm guard: "Tell me you've seen them. Those... things. Small and evil, like something out of a nightmare. They come from http://www.uesp.net/wiki/Skyrim:Stillborn_Cave, to the north. I know it."
Of course, they could be referring to chauruses.
Whiterun guard: "You know I saw something near http://www.uesp.net/wiki/Skyrim:Shimmermist_Cave to the northeast. Some sort of creature. Small and wicked, never seen its like before."
Windhelm guard: "Tell me you've seen them. Those... things. Small and evil, like something out of a nightmare. They come from http://www.uesp.net/wiki/Skyrim:Stillborn_Cave, to the north. I know it."
Of course, they could be referring to chauruses.
Oddly enough, I visited Stillborn last night and some of the little buggers caught me by surprise ... which didn't end well for me.
My current character is a Nord and I from that (taller) perspective, I could see where they might be characterized as "small", particularly as they always seem to be hunched over when walking.
There seems to be a nest of them fairly close to every city in Skyrim ... except for, oddly enough given the Chaureses wandering around, Morthal ... or at least, I haven't found any yet in the nearby vicinity. And if you're living in Markarth ...
While I personally can't stand the little creeps, I am curious about them, e.g., do they have any sort of social structure, a ruler, etc.
Based on at least one of them being able to control a Dwarven automaton and their ability to domesticate #%$#%%& chaureses, spiders,skeevers, I'm guessing they aren't as stupid as their primitive living conditions might otherwise suggest.
They definitely get out and about at times ... just haven't seen any outside myself. Doesn't seem like anyone else has either.

(would of got screenshot but on console
)