Mysterious wet spots

My son is a week old and today I noticed 3 wet spots on his bassinet mattress/pad and I can’t seem to figure out what they’re from. I don’t think it’s spit up because his face and clothes are clean in the morning and I also don’t think it’s pee because the spots are more where his head lays. Is it possible he’s drooling that much in his sleep or maybe he’s sweating? We do put the little beanie hats on him at night because we keep it cold in the house but maybe this is causing him to sweat. Ftm so this all new to me.