My 88 740 GLE was losing a lot of coolant recently, I kept filling the overflow
tank, a few days later it would be empty (car was running at normal temp). I could not see any leaks, and noticed the puddle was toward the rear of the engine, right below firewall. I checked the heater hoses, and sure enough the heater control valve was broken at the nozzle inlet feeding into the heater hose. Visually it looked to be intact, but was in fact broken clear through.
Later read that this is common on 740's (700/900 FAQ).

Jay
88 740 GLE 191k
84 245 GLTi 228k