hehehe ... I think I'll keep you in suspense ... it's not really here for artistic merit, but the effect is fascinating isn't it?
Perhaps should have posted it in a new thread so as to invite interpretation otherwise this thread could get taken over by guessing games ... so I'll put you out of your misery ... it is a glass door, and someone has stuck a glass tile to it, and then later someone has tried to break it off, leaving this shard stuck forever ... the door is open against the wall, and there is light coming from the right (it may be reflected light, now I come to think of it, the positions are all wrong for direct sunlight at the time of day it was taken) ... you can probably work out for yourself what that odd shadow is now ...