The only way air / moisture etc could apply the pads is if the return port to the reservoir won't open! which is like riding the brake pedal, or the rod from pedal to master is misadjusted holding fluid in the line and caliper. Air and or boiling water will make for spongy! or no brakes at all. Have you tried turning the wheel to check for brake drag? If you get em hot and the wheel turns with very little drag the caliper isn't in need of a rebuild. Make sure pads are centering on application, Bleed, and make sure the parts are together rite.
I have seen where either one can do what I said, because the reservoir is for holding extra fluid so as the brakes wear you will not run out of fluid.
I have seen brake systems develop air bubbles and I have also seen systems that have moisture in them. You can tell if the brake fluid has a lot of moisture, because if you have a site glass, the fluid will be a milky color.
Since both air and water will expand more and will do this at lower heat tempratures than brake fluid, this can cause the caliper pistons to push on the brake pads and pust the pads against the rotors.
You are correct about being spongy, this is also another side affect of having air in the system. Water can cause it to, too some degree.