I don't know if it was explained in the episode, but why didn't Shane have a tail following him that night? Who could have given him that kind of out?
When Vic finds out that Shane didn't have a tail he will put 2 & 2 together. Shane told Vic he was late because he was trying to shake off his tail.
To me it looked like Kav only had himself and that other guy from AI to run the tail.