Expansion tank location

I'm on a boat with year 1999 Detroits, 16 V 92's. The remote coolant expansion tanks are mounted in the bilge area, under the engines. I usually have seen them mounted on other boats, with other engines, on the bulkheads, higher than the heat exchanger. Why would they do these this way?

Pretty unusual, for sure. I mean, if it is not vented to atmosphere in some manner, then it can't really be called an "expansion" tank since it should already be full of coolant. How is the system vented, filled or checked? I would think that they would have needed to provide some sort of "snorkel" for these purposes.

The only other thing that I think would work for allowing for expansion is an accumulator. There would have to be a sealed chamber isolated from the coolant that would allow for the expansion to act on a piston/spring or a bladder/gas accumulator. That hot liquid HAS to go somewhere! Pretty fancy, costly and, at least in my mind, unnecessary.