I think Detective Holbrook could be Charles, I thought this awhile back when he was always very shifty and A was shown at the end to put in a password at the police station, a password that Holbrook would of course know. Then Hannah went to the warehouse with Calab and found the girls blood samples, and other items of the girls in police evidence bags, something else Holbrook had access too. Then he was at their winter prom and kissed Ali, but seemed shifty again. In season 5 when the girls make up a prom for Charles before shattering him with light bulbs you see Charles in a suit and mask (this kinda means when Aria says A is a girl that could be true but they wouldn’t be Charles) you see Charles has dark hair behind the mask, similar hair to Holbrooks.
Also why has he suddenly vanished? Charles/A have a lot to do with the police, they have the evidence bags, passwords, when the girls take their phones to the station they are wiped clean, Holbrook could of done this as he would of been there too. I strongly think we haven’t seen the last of Holbrook and has a huge part with Charles/A.
Theory by @EmilyCAndrews