Or its a special seal sensors can't penetrate.
Sides, all sensors are not Karin. They would see a normal Genin until the demon chakra started seeping out. I don't think all sensors are really that useful. Looking at how Orochimaru broke in, White haired dude from first episode almost snuck out with secret scrolls, Sound 4 broke in, Itachi walked in, and lots of Naruto Fillers shenanigans in which bad guys or people with special chakras went undetected until after they started causing trouble. (Remember that bomb one?)

No problem for Karin, but she's special because she's a powerful sensor.
Otherwise, I'd say the seal restricts enough of the beast that if Corrupt wanted it a secret he could run with it that way. Sensors are hardly the reliable sort that would know everything (like hidden powers, when a Jinchuriki goes on vacation, or be able to discern what a ninja is meaning to keep hidden)