Bit of a strange situation with our toyota avensis, would like some thoughts on. Its a 1.8vvti X reg with 80k on the clock. Runs sweet as a nut, starts straight up and returns close to 45mpg on a run, so to my mind the engine is sound. The thing is this, I checked the oil level today and nothing on the dip-stick

which is worrying seeing as it was garage serviced 1000 miles ago. I drove slowly home and decided to drain the oil to see what came out, not even 1 litre came out and it was as black as night. Also the drain plug was only finger tight. It took 3.5 litres of oil to get it up to the correct level. Now I either have a very sick engine that has used 3.5L in under 1K miles, (no visible smoke on start up, driving or when reved hard), a serious oil leak (not a sign of a drop under the car) or someone has been working on it, got called away and then forgot where they were or do you guys have any ideas. If say only a litre of oil was put in would the engine have survived doing 1K miles?