That's for people who try to gain touki without senjutsu. They usually go hand in hand as one cannot usually sense their touki without senjutsu. The only other way is to be extremely talented or just through effort like sairorg. Mc uses the A.I chip to sense minute changes in his body to sense touki similiar to senjutsu, but can't use it yet since he lacks the data.

Thanks for the heads up, don't know why the sub version of the anime said they were a marquis!

He transmigrated after Issei entered high school, and because of his lack of interaction with the supernatural until now, he hasn't had much time to discover the various energy sources. Though he will learn about them soon.