Strange Occurrence with my 2008 CX-9 Grand Touring

I'm going by what my wife described and what I observed afterward, but have no idea what happened. My wife started up her CX-9 to leave the house. The engine was cold but it was probably in the 70's here. She said she heard a loud tapping/knocking under the engine and when she looked in the rear view mirror she saw a lot of what she described as whitish smoke. No engine lights though and she turned it off. I came out about a half hour later it was smelling of gas so I had her start it up. I didn't hear any tapping or knocking and it was smoking out of the exhaust slightly but I couldn't tell if it was just moisture form being cold. The smell of gas and like something burning was very strong. As she said she saw white smoke and heard a knocking, I'm thinking Head gasket so I had it towed to the dealer. They called me today and said they could find nothing wrong. No engine codes, nothing. I went and picked it up and drove it home and it runs great.

My wife swears it was knocking loud and her rear view was full of smoke. She said she never touched the gas and cranked it like always. It has 125,000 miles on it but has never given us a problem. So while it runs great now, I'm having some trust concerns with the car as my wife takes my daughter to a lot of doctor appointments. I would feel much better if I could get an idea what happened and if its something I need to worry about. Anyone have this happen before or have any idea what it could be?

Thanks for any insight.
 
Have you checked the oil level since this happened? Also, is there coolant in the revovery tank for the coolant? The reason I am asking is that the water pump failure would cause dillution of the oil and perhaps lead to the knocking noise. I cannot account for the smoke your wife observed but I think a check on the status of the oil and collant could not hurt at the moment. I do know that if you drive for too long with the contaminated oil the engine will be toast in a short time.
 
Oil level was fine and I didn't see any discoloration of the oil. In the troubleshooting checklist they marked all coolants as ok in looking for possible problems. I had them change the oil while it was there and they didn't note anything. I will check the coolant levels tomorrow and oil again to see if anything looks off.
 
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