Force signiatures can be hidden, like by making yourself small. Usually the footprint is not that east to read, unless it is a place strong with the force. But in places like the crystal cave where the whole area is filled with force resonating crystals and what not, it is near impossable to hide your footprint.
Thing is, Jolee thinks Revan at the time is somewhat grey like him, and it is easy for him to go one way or another. So he decides to help, well, at least to stop him from going too dark.