I see now. One of the advantages of using refrigerant is that it leaves those white traces to help with identifying leaking points.
The cap can be an issue, true, but before that check the connection between hoses (2 in/out) and radiator as they tend to drip. I see as “good news” that you see the water accumulating just under the radiator and not deeper behind which could mean a pump leaking (worse news!). Also, be aware that if you fill it up completely when cold it’s normal to have a puddle because the excess has to leave the circuit through the little hose next to the cap once it expands at hot temp. That’s precisely the job of the spring-loaded cap and the little hose next to it. Just in that area, check also the nipple that comes out of the neck of the filler opening, they get loose and they leak too. This is less critical as it’s refrigerant at atm pressure not part of the cooling system but simply letting the excess drain out.
Hope it helps.
Cheers