If it goes back to the dealer again, make sure they also check the pump. They may just be looking at the tank and saying no leak but if the car is being driven and the pump is leaking, it may evaporate once it drips which would explain why there's no evidence of it. Also, if you have 2 people present, have one sit in the rear (behind the back seat) while the washer is running. Make sure the rear washer hose/connection isn't leaking into the vehicle. The interior materials would hide the fact that it's leaking. Keep us posted.