The only thing I can think of is an Easter Egg left by the previous owner. Maybe he (or she) was trying to suck fuel out through the filler cap at some point in time, dropped the hose in there, and couldn't be bothered to fish it out?
There's nothing even remotely looking like that hose in the drawings of the fuel pump or fuel tank.
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Suzuki DL1000 V-STROM 2005 Fuel Tank - order parts in just a few clicks from Europe’s number 1 genuine spare parts online retailer
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(#11 in the last link is the drain hose that connects to the copper vent line you saw running through the tank. This hose ends underneath the engine and is just left open. Plus, that hose is black - at least on my bike.)