I’ve thought that he’s just been hiding out in SB, likely at the convent with Blue. He may have feared Rumple. If he expected Nealfire to be coming to town, he may have been wary of what his reaction to him would be. Nealfire’s presence in town would also likely cause Emma to be hating on August for having broken them up. He was certainly ashamed to be face to face with his father now that Geppetto remembered everything and knew that that guy who had been working with him, who hadn’t done his father proud, was his son. So I think there’s ample reason for him to have fled to the one person who already knew all of his secrets and hadn’t disowned him: Blue.
Like SpinningGold said, there is an upcoming ep where we’ll finally be getting some August answers. We don’t have spoilers confirming where he’s been all season, but we do know that something significant will happen concerning him.
As for the question of how August knew about Bae, there’s a whole thread discussing that here: https://oncepodcast.com/forums/viewtopic.php?f=70&t=2941 🙂