If you don't have your truck leveled, the rake is going to force the majority of the water forward, even if it came from the rear of the truck. Typically you will see moisture at the back when this happens, even if a limited amount.
It helps if you can catch this while it's raining. My last truck had several leaks, minor, but they all collected in one spot, away from where the leaks were at. I didn't figure it out until I used a sprinkler on the truck and then opened the bed to see where it was seeping in.