I had the same problem. It leaked enough that I'd top off once a month, then suddenly I was topping off every other day - then it just dumped it all out.
Ended up being the return line to the cooler. Pain in the ass to change, and though I couldn't find any holes on the old line, it was rusty as hell. I believe the lines (came with both lines for the cooler) ran me about $42 from the dealer.
Try wiping everything down on the underside, and then look for the source. Mine was a pain because it was leaking in a dark hidden spot just above the subframe.
FYI: I ghetto hacked it when I put the new line on. I couldn't figure out how to get the fitting off the rack, so I chopped the old line about 20cm from the rack fitting and butchered the same end off the new line. Use some hi-pressure hose with about 1cm of slack and very tight hose clamps if you decide to go this route. (better if you don't have to)
EDIT: I was a disgusting oily mess after doing this. Totally ruined the fleece I was wearing and left a very large spot on the driveway. One you cut that line, it just pisses all over the place.