Oil smoke from under hood 240D

[TedG]

I was driving pretty much flat-out (about 85 mph) along the Interstate one morning last week. After about 25 miles, I exited. When I stopped at the the top of the ramp, quite a lot of acrid blue smoke was coming from the sides of the hood. I shut the engine off and popped the hood. The smoking had stopped, but I was concerned. I checked the oil and it was low; just a bit on the end of the stick, but then I had just keyed off. I had to add about 1 & 1/4 liter. I confess I hadn't checked it for a couple weeks, so it may have been down a liter or more before the smoke. There's no sign of significant oil leakage on the engine and no oil splatter in the engine compartment. Oil pressure and coolant temp. were fine when I saw the smoke and have been fine since. Ambient temp. was about 55F. I haven't seen any smoke since, but I haven't exceeded 70 mph since, either. Any ideas where this smoke may have come from?
Car has 90+ k miles on it and uses maybe 1 - 2 quarts between oil changes (every 3k miles).