White under radiator cap

Takes more than a 3km drive around the block to work that out.  Keep an eye on the coolant level every few days and top up as required.

Condensation that occurs naturally during heat cycling (i.e, cold, hot, cold, hot, etc...) will collect in there.  If there is any sort of residue or slime in the cooling system, it will commonly surface around the filler neck and cause all kinds of funky colours to show up.

You can just go an buy a 1L bottle of pre-mixed coolant and keep and eye on it, 
 
White under the oil cap is just **** in the coolant, what you really need to keep an eye out for is when you have white under the oil cap.

Pictures woukd help a lot here. Where is it located on the cap, Is it powdery, Is it slimey/oily, etc. etc.

once its gone white and 'sticky" its a sure sign of an unwanted reaction, usually combustion gasses in the water from  a blown head gasket.
a wet look is normal, but you cant always see a color from the coolant used. sometimes however its really obvious if blue or red coolant is used as the underside stains. i have been caught before with the overflow bottle filled with oily gunk from prior failures and
never cleaned out, and that can somethime slowly eeek back into the system and stain the cap.
 
have you checkes under the oil filler cap for any sign or condensation or 'whiteness". any "whiteness" there is definitely not normal and another indication of collant/gasket fault
 
Another posisble problem is diffenrent collant being mixed and reacting. the results are unpredicatable, but sometimes this ends up as a slippery mess under the cap and right thru the entire system
my suggestions are:......
remove and thoroughly clean the oil cap, over flow bottle, filler neck and rad cap in hot soapy water
top up and bleed the water sytem as per factory instructions. dont worry about any coolant/inhibitor
test drive till it gets real hot
let it cool and check both caps..  moisture or whiteness under oil cap are NOT normal.