your post is a guess based on what you think was 15 pounds of pressure when you turned the rad cap to remove if i read it correctly < a normal cooling system will maintain its pressure for a few days easy and most systems run right at cap pressure so yes that is normal

if you have a 14 pound cap and it has over 15 as a guess by the amount of air blown out at removal than the 14 pound cap is junk and not doing its job, the level will move up and down in the expansion tank as it should cold to hot and back --

20 DEG and seapage might be a thing i would run the car for a 1/2 hour on the road than re tighten the hoses that are seaping while they are hot and soft the clamps will just bend on very cold hoses and make it worse THAN see what happens -- jz