I was checking things on my car the other day so i took the rad cap off to check the level and test the antifreeze. The car has not been started in over a week, but when i got the cap off, the rad. let out a wheeze of sorts, like it was pressurized? What gives?

If it matters my mix is like 25%coolant/75%water.

 

It usually has vacuum... that's how the recovery bottle works.. the water gets hot, expands, forces any exra out into the bottle once the radiator cap's design pressure has been reached; the radiator cap has a one-way check valve built into it, such that pressure inside the system closes it; when the coolant gets cool it contracts, and draws some back in from the bottle through the check valve in the cap; the check valve requires some amount of pressure differential to work it, so a small vacuum remains in the cooling system if there's no leaks.