Weird cooling system issue

[trac209]

I'm trying to track down an issue I've never seen before. My cooling system even cold will seem to build a slight pressure(just enough to let some coolant out when cap is released). I thought maybe head gasket was letting combustion gasses into the system. So I first checked all plugs,all look great. Then I pressure test cooling system. It holds pressure and vac. Ok now I'm stumped. The truck had the rad cap off for a day while I was doing other stuff,the spark plugs were out too so there wasn't any compression in the engine whatsoever.

So I put the plugs in and capped the rad, did not run it and came back to it couple hours later and pulled the cap off and a bit of coolant comes out again??? The truck never runs hot whatsoever runs awesome but I've never seen this happen before,at least I didn't notice it before. I can't understand how it pops a little bit of coolant out when it could not possibly have and pressure source to do so due to it not even being turned over let alone run. I'm stumped. The only thing I can think of is some sort of chemical reaction in the coolant???
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