Had a Madrone pee all over me - must have been 20 gallons of bright red water in that hollow tree, - thought I had cut myself bad for a second! HERE is a web page that discusses trees producing methane. They have a video of a tree out gassing methane, which they actually lit. DO you feel just about like you've seen a big foot? Even if it wasn't hollow, the methane gas likely built up between the cambium and the bark - I believe you. Oh! And I'll wager the gas vapors drifted up rather than down, as Methane is lighter than air.