The light is telling you you're low on coolant. So you might want to get some 50/50 mix of MB coolant and water and refill the expansion tank.
The hose you're following is most likely an overflow hose for either the windshield washer reservoir or the coolant expansion tank. Either way, one end of it is *supposed to be* disconnected.
Regarding the cap type: If it is a snap-on type cap, then you're found the windshield washer reservoir. On all my cars the cap is imprinted with the windshield/squirting water icon.
The coolant expansion will have a screw-on type cap because it is required to hold pressure. It will also have the same symbol on it as your coolant warning light. There will also most likely be a warning sticker telling you not to open it when it is hot. Good advice. I suggest you follow it.
