Does anyone know what this symbol means or hints to?

[img]http://i953.photobucket.com/albums/ae20/phoenix40k1/ScreenShot17.jpg[/img]

Once you join the thieves guild you'll find a book explaining this and other symbols like it.

The book is called shadowmarks. If you don't feel like waiting to find it here's what all the symbols mean http://www.uesp.net/wiki/Skyrim:Shadowmarks

I see that on doors all the time; no clue what it is.

Shadow mark meaning there is good loot in that place to steal.