I, Mike, decided to fly to a local airport that has a large pilot's shop to pick up some airplane wash supplies. On base, smoke was coming in the defrost vent on the passenger side. The smell may have been electrical but couldn't be sure. It didn't quite have the classic smell of burnt wire. I immediately shut down the A/C. The airport was busy as it was memorial day and a great day to go flying. I made my short final call on the radio and informed the FBO of the problem. Then switched off the avionics power and the master switches. I was able to landed without incident.
On the ground, the electrical system and avionics were checked. No systems were found inop. No breakers were popped. I removed the cowling and noted some oil around the pilot side front cylinder (cylinder number 6, I think). I checked the oil level and found it to be in the proper range. I ran up the engine on the ground. All systems were still OK. No more smoke was detected. I made the decision that my passengers should drive home, but to wait a bit for me to take her around the pattern. Ultimately the plane and I flew back home.
More details to come after I hear back from my mechanic.