My bike does this every couple of years after storage over winter. I have found that the float obsorbs some fuel and thus the level overfills. I have adjusted it some (hard to get right on mine) but the best bet for me was to replace the float.
Either way, it should NOT drip or leak fuel. I have not seen any BMW at the shop I worked at leak except as I described above.
Oh, just to be clear, mine is an R60/6, so a bit older - and it has a rigid foam float as a stock item. If yours is a BMW it should have a petcock - I make a habit of closing mine after every ride as sometimes heat changes will cause fuel to leak.
I don't park on the side (ride off kickstand keeps it level) but I have a friend who does sometimes and it will occasionally do a small drop if he does not close the petcock.
EDIT NOTE: ensure that this is NOT from a cracked fuel tank.
You don't say if this is an R series or the F series (looks like an F series from the picture though). I think there was a recall on the F series tank at one point.
Also note the Fuel Injected bikes (GS, Dakar, CS, G650X) have pressurised fuel lines. When you turn the key to the on position, you should hear the fuel pump pressurising the line. Just after you turn the bike off the fuel line will still be under pressure.
Is this leak just during riding or while it is stopped? Will it leak just after you stop also? The pressure might last quite some time if that is the case and a small leak might be detected for a bit after you turn off the ignition. You might need new clamps and a fuel line in that case. I would recommend having a qualified service tech replace those in that case.
Try removal of the seat, looking at the fuel system with the key on to see if you see leaks/fuel pooling there as that might also occur - keep in mind the fuel is pressurized for the injectors.
Final note: there have been some fuel system recalls on the F series - you can search for those with your specific model number as well.