oil leak? 94 E420

crap.. just picked up my car from a dealer after a regular 3000 mile oil change. the service advisor said that i have an oil leak. the car has only 42K miles. i bought the car when it had 38.5K miles and i had the oil changed by the same dealer then. at that time, the mechanic told me that the car was in a great shape.

so my questions are:
1) is this a common problem with this engine/model/year?
2) is this a serious problem that needs to be dealt with right away?
3) how much should i expect to spend?
4) how do i know if the dealer is telling me the truth? after i drove the car back home, i looked at the driveway but there were no oil marks on the pavement.

however, i have noticed that the low engine oil indicator comes on when i push the car hard when the engine is cold. it would be one for maybe 10-20 seconds then it goes off. once the engine is warm, the light doesn't come on regardless of how hard i push the car. i didn't think much of it. is this a sign of an oil leak?

please help. when i bought this car, i was expecting a fine mercedes experience. i've had the car only for 5 months yet it has given me more problems than my 4 year old honda.