I've been noticing that some buildings have a small circle with a square inside it, some of these squares have lines through them, others don't. I'm curious about the meaning of these. If it's quest related or whatever, then don't tell me what quest, but I'd like to know one way or the other so I can stop worrying about them.

It's Thief Guild markings. Each symbol has it's own meaning (treasure here, danger!, etc)

I saw those markings also in dungeons..

Now im getting curious, i think i might join the Thieves guild after all.. Though my character is a good the good one, and i hate the thievesguild..

Ty for the info.