I have unexplained coolant loss in my 300E. I show no evidence of a leak (residue, puddles, oil contamination, etc.). Before I drop in some Stop-Leak material, are there known areas that I should be aware of?

Coolant leak as you describe is typically associated with a head gasket leak. Look (with a flashlight) at the passenger side near the firewall of your engine block and you may see white residue from your coolant leak streaming down.
There are plenty of threads on this repair, as it it is quite common.

If it is your head gasket, you'll start to see oil in your coolant overflow tank.

Even worse they may be coolant in your oil sump.
If your car is hard starting with an uneven hard idle when cool check the air filter and make sure there is no residue coming in from the breather tube.

I see no evidence of a leak. Engine oil OK - no froth or water in bottom of pan. Engine runs normally with good power and torque. It could be escaping through an exhaust valve, except the exhaust does not seem moist. No residue near head. Still trying to figure it out.

Maybe it ended up in your overflow tank located in the right fender. Maybe you have a bad cap or something and it boils over into that. But not so much as to overfill the overflow tank.