2000 VW Passat V6 MIL

You have a heated oxygen sensor. In order for the oxygen sensor to work efficiently and quickly after startup, a heater is incorporated around the oxygen sensor. It's not a major fault. Though just the same check the wiring between the sensor and the computer just to make sure everything is shipshape. Unfortunately the only fix might be that you need to replace the oxygen sensor even though the sensor itself might still be good. (It's all one unit)

It's good practice to replace the sensor occasionally because they do wear out. Every 50,000 - 80,000 miles or so.

The reason they asked if you had gotten bad gasoline or if it had been running rough was to pick your brain for possible reasons the oxygen sensor itself might be going bad. Or telling the computer to set a fault code. Bad gasoline will cause the engine to run less efficiently, and the oxygen sensor might complain about that. Doesn't mean the oxygen sensor is bad, but it might set a code anyway.

From what you've described, it doesn't sound like the oxygen sensor itself is going bad, or that it is complaining about some fuel trim parameter that is out of range. There are other fault codes that would pop up if that were the case. So you car sounds like it's functionally in good shape.

It sounds like the car's computer wasn't able to verify that the heating element that is around the oxygen sensor is working correctly. It's either gone bad, (in which case you'll need a new oxygen sensor because they are both contained in the same unit) or the connection between it and the computer has become compromised.

The oxygen sensor unit tends towards sensitivity and fragility. Partly because of it's environment. High heat namely, and the substance it needs to monitor constantly. Oxygen sensors never last the entire life of a vehicle. I hope this helps.