I'm wondering what happens when H2 leaks from say a transmission pipe in an unenclosed area. No immediate source of ignition, I know it rises more quickly than helium (x2 I believe) and dissipates in concentration quickly but does it react with gases in atmosphere to form methane or other compounds?

Short of ignition and burning causing more H2 to dissociate what can happen, especially in upper atmosphere with more UV and so on. Methane will react in the upper atmosphere subject to UV radiation giving it a half life of ~7 years for example.

The covalent radius of a neutral hydrogen atom is 0.0371 nm, smaller than that of any other element. Because small atoms can come very close to each other, they tend to form strong covalent bonds. As a result, the bond dissociation enthalpy for the H-H bond is relatively large (435 kJ/mol). H2 therefore tends to be unreactive at room temperature. In the presence of a spark, however, a fraction of the H2 molecules dissociate to form hydrogen atoms that are highly reactive. source chemed.chem.purdue.edu