quick question on coolant reservoir anatomy [85 300d]

[ar300d]

I've been told once or twice its nothing to worry about, but there is a thin hose that goes from just behind the coolant reservoir cap and lays down on top of the wheel well. In my car the end of this hose is frayed and open, and I cant locate anything else with a frayed/open end nearby. I've read this is an 'overflow hose' but I'd think it would dump coolant on a good hill and allow water to become steam and escape over time. I know on some later mercedes models there is an 'overflow' tank connected to the reservoir, but I dont think my 300d has one.

Does this part of the coolant system have to hold a lot of pressure, as much as the rest?

Then again I opened the reservoir cap earlier, it hissed for a second, so its holding some amount of pressure.