A leak detector assembly for use with a backflow prevention device includes a housing defining a passageway for receiving a fluid. A flap assembly couples to the housing for generating a low flow error signal indicating minimal fluid passing through the passageway and a fully actuated error signal indicating significant fluid passing through the passageway. The flap assembly includes a flap mounted in the passageway such that significant flow of the fluid moves the flap to generate the fully actuated error signal. The flap assembly also includes a sensing element on the flap to determine a presence of the fluid without movement of the flap to generate the low flow error signal based on a low flow of the fluid.