Why does Percy begin to suspect that Ares wasnt acting alone that he was taking orders from someone?

Because Percy cannot understand why Ares would steal the bolt only to have him deliver it to Hades, he begins to think that the creature or being in the pit actually masterminded the theft.

Because when Percy questioned him, he wasn't paying attention and was in some kind of trans (I don't know how to spell it) and when Percy asked what that was about, he said he didn't have dreams and then Percy said that he didn't mention any dreams