I've never actually seen the recovery tank in my 190E, but there is a plastic splash panel at the rear of the wheel wheel, so since I'm about ready to do a 15K maintenance and a coolant change, I think I'll remove the panel and have a look see.

As the coolant heats up it expands and compresses the vapor dome in the pressurized supply tank. If the coolant gets hot enough to form vapor bubbles in the block or head the 15 psi pressure relief valve will open and vapor and maybe some liquid coolant will be expelled. The vapor is some combination of air, water, and glycol and the latter two components will cool and condense to liquid. Of course, any air will escape. If the system develops enough vacuum as it cools to open the cap vacuum relief valve, the condensed water/glycol mixture from the recovery tank will be drawn back into the system. This way coolant is not lost if it is temporarily expelled from the system due to temporary high operating temperatures, which is what would happen before the general adoption of coolant recovery systems.

On my car the hose disappears behind the inner fender to the mysterious recovery tank. I usually apply vacuum during a coolant change and just get gurgling, so there is very little liquid in it. I then use a thin neck funnel to pour about a pint of water into it and then siphon it out. As I said previously, the liquid appears clean and free of debris.

Duke