Please bear with me as I tell my little story, because I'd really appreciate your insight into this matter: two mechanics are telling me contradictory stories and I don't who to believe.

About three weeks ago, I brought my 2001 V4 Passat sedan to a local mechanic for an oil change. I ended up with $1600 of work (tires, brakes), but that's another story. Three days later the dashboard flashed a warning, "Stop. Check Coolant." So we drove it straight to the nearest Sears, where they referred us to a VW dealer because the Passat requires special coolant that they didn't have on hand. By the time we got the car to the dealer (the next day), the engine was smoking, though the temp gauge still read normal.

The dealer said someone had put the wrong kind of coolant in the engine. He showed us that the coolant was a brownish color when, he said, it should have been red. He was very confident someone had put in the wrong coolant, and said it had to be flushed.

So he flushed it, but he also did a small repair on a leak in the "water pipe from the block to the heater core hose" (quoting receipt).

So here's my question: can I reasonably conclude that the first mechanic put the wrong coolant in the engine? No-one else serviced the engine, even for an oil change, for several months previously. Is there any other reasonable explanation for what happened? Could the dealer's mechanic be wrong about someone putting in the wrong coolant?