Identification of a small flat bug
Definitely an arachnid and mite (subclass Acari), and very likely a member of the order Parasitiformes, of which there are more than 100,000 species!!

Your specimen brings to mind the family Argasidae (the "soft ticks" -- so called because they lack a hard scutum).

Specifically, your specimen reminds me of Argas reflexus, the pigeon tick. (which has been found to be an increasing pest and human concern in urban settings).



 Credit: Milan Kudlička  

Based on the size of your specimen, I would guess you're looking at a non-adult stage. 

However, this is just a guess, and I will try to explore other species in this general group to see if I can come up with a better-supported guess. 

(By the way: any pigeons living nearby??)