That is not a leak. The is applied at the factory. Not exactly sure what it is for but it is used on the body seems. All vehicles have it. Now if there is moisture in the trunk I would first check the rubber seal. You can fold the seats and climb in while the truck is closed. Have someone run the hose around the lid and you can check with a flashlight to see if any water penetrates the seal. If all is good you can check rear lights, license plate lights, any place where water could get in to see if they have a good seal. If lights are good then you can peel the carpet back and remove the side trim to find where the, I don't know the technical term, rubber flap. There is a trim piece with a rubber flap that allows the air to escape the vehicle when doors are closed and also to allow the air to flow through the vehicle. Sometimes the rubber can degrade, warp or rip and because it is usually behind the rear wheel water can get in. I know this because I have experienced it before. If none of this is the issue then rear window seal check.