The theory, posted to the PLL fandom wiki, posits that Caleb is actually Charles, which in turn also makes him A.D. Basically, it states that Caleb's childhood in foster care could easily line up with Charles' past, and the fact that Caleb was housemates with Charles' best friend Lucas adds further proof.
The poster also points out that A.D.'s game seems to obsessively target Hanna, which could be because we know Hanna and Caleb broke up in the 5-year time jump. Check out the full theory below:
It's actually a pretty convincing theory, especially considering Caleb is definitely the most technologically competent character on the show, and he has the money to do all the things that A.D. has been capable of.
The one possible hole that a commenter points out is Caleb seems too young to be Charles. Then again, Caleb could have been lying about his age this whole time and just is lucky enough to look much younger than he is. Hey, weirder things have happened on this show.
We'll just have to wait until the big finale next week to find out for sure!