I drive a 2006 F150 w/ 5.4L engine. Every now and then my windshield gets a misty spritz of fluid on it. I'm assuming that the fluid is coolant because instead of evaporating it drys into a crystalized mess. I don't have a problem with overheating but I do see a loss of coolant when I check it every few months. Today I did a pressure test using the fill hole on the reservoir tank. This appears to be the only spot to connect a tester.

I pumped the system up to 15.5 psi and let it sit for 2+ hours while doing other things. After 10 minutes there was a tiny drop but still above 15. I checked it every so often and the pressure continued to drop. After more than 2 hours it had dropped to 12 psi. While test was occurring, I checked the system for any evidence of leaks and found nothing. Nothing on the ground either. But I know antifreeze has accumulated on surfaces on, beneath and around the cowlings, which are located at the bottom of the windshield.

I'd appreciate any suggestions on what I should do next.

Note: I don't believe the spritzing on the windshield is due to direct spray from a leak. I'm fairly certain that the spritzing occurs when I get up to speed and coolant that is sitting atop any surface is blown back onto the windshield.