I've had trouble in some cases locating external cooling system leaks on my MB.

The only way to know for sure where it's coming from is to have the cooling system pressure tested. You can hire a shop to do that or rent one from one of the mass-marketers like Pep Boys. If that test doesn't expose the coolant loss, then a cylinder leak down test would help to oncover a leaking head gasket.

Good luck.

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'91 300-SEL