Why does the sunroof on my 2001 V70 open by itself after the engine is shut off and the key is removed?

I purchased my 2001 V70 about 2 months ago. It has 80,000 miles on it and runs like a top. It does have one strange problem from time to time. The sunroof will open by itself after the engine is shut off and the key is removed from the switch. It even happened once, as my wife was driving slowly in town. I have returned it to the used car dealer where I purchased it, but the problem never happens when it is there. He drove it for several days and naturally nothing occurred. It may go for two weeks, then open several times in a row then not again for a length of time. Any ideas as to what is causing this? Can I disable the sunroof easily so it won't happen in the middle of one of our soon to have snowstorms? Thanks for any suggestions.