Spitballing here chico, also kidding around, but I really do like Pyro's idea. Energy reading + spirits was a go to for me because of that little moment when all the spirits were like "lol cant sense raava not avatar ya" in Korra, but the guru might be a better idea. Maybe he fell from his pacifistic ways because of something traumatic and has turned to seeking power as a way to cope, or he is being forced to help out the Seekers by blackmail or some other means.

It's best to assume, also, that the Seekers were ridiculously outnumbered in earlier years. It might have been just some crazed leader and a colleague or two. They grew with time. It has just been a year or so since rumors about them have started appearing.

The Jiyu also get all their info from multiple sources. Baited gang members, spies, anything that will work with them is a source of info. They could easily find out about the sudden capture of an unassuming, young firebender.