My 2013 - 90 year anniversary edition R1200RT fuel gauge slowly went from an accurate low fuel level reading with 71 miles left - up to a full tank with 240 miles left, on my way to work yesterday. This took about 30 minutes to do this. When I left work, the gauge was still about full and I stopped at a gas station for gas. Opening the fuel cap usually is just turn the key and flip the cap up, but this time it took more force than normal and I heard an audible 'WHOOSH' as the vacuum in the tank returned to atmosphere. Does anyone know how the tank is vented and where I should look first? For all I know, this could be electronically vented per the 'brain' or as simple as a fixed, hard pipe vent. I'm at a loss on how the bike ran at all. I'm thinkin the fuel pump has enough push to overcome the vacuum that happened. I've had many bikes, but have only had this 7600 mile bike for a few weeks and this is my first BMW. Not as quick as I like but a very nice bike I enjoy more and more. Anyone pointing me towards fixing this is much appreciated.