Hey Paizo Gurus!
I think I found something left out of the bestiary with regards to the Doppelganger. The stat block lists (SQ) change shape (alter self) under statistics, but the Special Abilities block only lists Mimicry (Ex) and Perfect Copy (Su).
I think this got left out by accident? After all, alter self by itself has a time limit (1 min/level) and I'm betting that the Doppleganger's is at will. I'm also betting the Doppleganger has it 'at will' with a 'one turn' requirement to change shapes, but none of that is specified. I'm also wondering if a true seeing spell would allow them to be seen normally. Again, just wish the ability had some details to it. I'm sure this is just a typo.
Just to note, I did follow the link down to the 'shape change' ability, but that also doesn't list it as being at will, or how long the change takes, or whether true seeing shows their true form.