For the past few months I noticed a decrease in my coolant level, so I had to top it off every 1,000km or so. Out of curiosity (and fearing the worst) I sent an oil sample to Blackstone Labs, who called today and said they found traces of coolant in the oil. Not high enough to break it down, but present nonetheless.

I just bought this car so I never changed the oil. Judging by the service interval, this oil has been in the engine for 9000km.

How do I locate this leak?

In addition, the plastic coolant container (located near air filter) had a leak in the past, because it corroded the body right underneath it. I think that's not an issue anymore, because the cooling system is under pressure. At least that's what I think because whenever I open the cap there's a "hiss".

Any suggestions would be highly appreciated. I'm kind of worried.

The engine is OM606, a turbo-diesel.

When running hot, there's no smoke, unless I floor it really hard.
3 GPs are burnt, so when I first start it there's smoke for 30 seconds, after which it runs very smooth.
City gas mileage is 10 l/100km... not sure what that is in miles/gallon. Sorry - old European habits.

Thanks for reading and for your feedback!!!