Ok i've been looking into this completely in game for a while now and nothing has piqued; so i'm bringing it here.

There are these markings, seemingly random, notching into the wall next to doors outside of taverns, residences, shops in every city and town i've come across.

It looks like a circle with a square inside, which is further divided into three sections horizontally.

Like, what is all this about?

I'm pretty sure this has to do with the Thieves Guild..

I've only seen them in riften?

http://www.uesp.net/wiki/Skyrim:Shadowmarks

Thank you, perfectly put.

I'm in Riften all the time and didn't even notice these. This game fascinates me.

Quest for Glory IV (an adventure/RPG with a leveling system virtually identical to that used in TES (also my favorite game series ever)) had a system of thief marks that it used in a few areas to tip off thief players that there might be something interesting nearby, and when i saw that an extremely similar thing was introduced for Skyrim i grinned like an idiot and couldn't help but wonder if the Quest for Glory series was actually as influential in the development of TES as i feel like it is (cat people living in the desert OH WAIT).

i mean i get that QFG4 didn't invent the concept of thief marks and they're a much older thing but it's super nice to see something like that and think of the possible connections between the two, and even if they're completely unrelated the fact that Bethesda did something as [censored] tiny and ultimately insignificant as devise something like this knowing full well most players would never notice it at all is so astoundingly refreshing.