I'm a bit late on this but, it is definitely the evap system test. Basically, what the computer is doing is running a little compressor that pressurizes the fuel tank with air. It records that pressure, and then a little while later, records the pressure again. If the pressure has dropped since it ran the compressor, it knows that there is an evap leak somewhere, and will throw up a CEL.
This is how the computer knows that you 'forgot' to tighten your gas gap when filling.