it's fault of the subject
Girfriend: You was with a girl on skype yesterday, I saw!
Boyfriend: Indeed. But you was talking with a guy in chatroulette too, so it's on you.
To act sneaky and secretly without other peoples know about it. To act furtively.
Guy 1 - Have you seen those news about the mobsters of L.A.?
Guy 2 - Oh yes, I have. They were finally discovered, working undergrounds.
Guy 1 - Haha, they were flying under the radar of our city police, but sooner or later they would be found.
Guy 2 - To fly under the radar is no easy deal, here in L.A. ... One hour or other they catch you.