Head or Lower intake gasket?

Hello, I have a 1995 V6. The coolant in my car slowly disappears. It is not leaking on the pavement, its not in the oil, and it does not look like its coming out of the exhaust but I suspect it is. I did a pressure test (used a vacuum and gauge on the radiator) and the cooling system held pressure for about 20-30 seconds then very slowly started dropping.

My uncle (a mechanic) says based on that information he would suspect a lower intake manifold gasket. What do you guys think? I haven't done a compression test yet but that should be the determining factor between the two right?

Thanks.