If there's no coolant in the oil then maybe if you put a white sheet on your drive way and parked your car over it, started it up and ran it for awhile maybe you'll be able to spot some discoloration.
I don't know.....just an idea off the top of my head....Shurlok, Jr. here!
Another trick is to clean all your hoses, then start your car and use paper towel to go over the fittings, hoses, etc. to see if anything is picked up.
How about heater hoses?
2004 E60 530is, Delivered 12/12/03, TiSilver/LGrey, PP, SP,PS, PDC, ACC, FRS, Step, Xenons, clear bra, body colored rear inserts. (Retired, 1983 henna red 320is)
2004 Corvette Z06 Commemorative Edition