Is your sensor positive or negative pulse? If you have a bad sensor and leave the sensor wire “floating” all sorts of wierd things can happen. The wire acts as an antenna and once the voltage on the wire reaches a high enough peak it will trigger a signal. The wire can get interference from just about anything, like wireless routers, your TV, the microwave, or even radio stations. So trying to tie the phantom pulse to something can be very difficult, however, it is likely you have a bad sensor and replacing it will remedy the problem.
If it’s a positive pulse setup don’t ever leave the wire hanging in the air with nothing attached, a large enough charge can build on the wire and damage the port. This is why cable TV ports left open will have terminators, a basic load, on them.
If you have a negative pulse setup, watch for bad or disconnected grounds. A partial ground fault can cause spurious signals. Leaving the wire hanging in this setup usually results in bad or no data, but can damage the port sending the signal down the wire by acting as a short circuit. Odds are this is the type of sensor you have and it is either bad or the wire going to it is damaged or improperly connected.
Short answer, you most likely need to replace your flow sensor and the problem will go away. Hope that helps a bit.