Chiyo has always treated Takeru as her father in a way, since that makes sense that Takeru might be her father.

The attempted possession took place while Chiyo was distracted, being social with friends. And then said Ganma realized that there was a Ganma soul already in there that felt strange. Riko's aunt has told Chiyo she's special, but before the whole occurrence with the Ganma Chiyo has just assumed that's a compliment.

Which was why Alain showed up at the temple in the first place. It was complete coincidence that he followed Chiyo and Riko in when he intended to look for Makoto and Kanon, and Kanon didn't precisely tell him that one of the girls that had appeared just before he did was the one he was talking about/looking for.