[Sorry, I somehow managed to reply to another post (system has been so slow recently), so I'll copy my reply from yesterday up to here in case it helps:]

Sure does sound suspicious, but not as suspicious as it could be.

There are a numnber of ways a head gasket can leak
https://haynes.com/en-us/tips-tutorials/7-different-ways-head-gasket-can-fail

Loosing coolant out through the exhaust pretty much means going through a cylinder unless it can somehow get out behind an exhaust valve. One of the plugs usually looks different and soon becomes spotless of carbon deposits after repeated steam cleanings. Your observations don't confirm that, so I think that raises major doubt and your head gasket may be okay.

Even more to the point, if the low pressure cooling system is leaking into the higher pressure combustion and exhaust system then as soon as the engine gets really hot the coolant will turn to steam at the leak. Some may come out the exhaust, but at the same time the cooling system pressure rises, the level will suddenly start to rise in the reservoir and may vapourize or even leak out under the rad cap. If you can run virtually forever with the rad cap loose then this isn't what's happening, in fact that's one of the diagnoses I was taught for a coolant head gasket leak in our red blocks (with the rad cap off, idle until the thermostat opens and keep going for another 15-20 minutes with the rad cap off, there should be no boil up).

If the excessive smoke out the tailpipe is basically white then that can be normal under certain conditions. This is the time of year where damp weather allows condensation to form and even accumulate in exhaust systems, especially if you park outdoors overnight, and especially if you do a lot of short trips. 240s (and 140s) with a rear muffler across way at the back are horrible for not getting hot enough to evaporate condensation in the rear muffler. If you do a lot of short trips then condensation can build up to the point it's dripping out the tail pipe onto the drive, especially when parked on a slope, and it will be rusty as the muffler and pipes start to go. Any of that sound likely in your case? I try to make sure my tailpipes hang down at a slight angle to help with draining. I figure better to hang low and risk taking a knock than rot out quicker.

Barring other symptoms, it will take compression and leakdown tests to do a more formal diagnosis of the head gasket.

If the coolant level is dropping slowly then there are a number of possibilities where it may not be the head gasket and you may not see evidence on the floor. When was the last time the water pump was changed? Is the rad cap known to be good? Any hint of stains on/under the rad hiding behind something? Any hint of a sweet smell in the cabin?

After a good trip and the hot engine gets cool enough to touch (but not enough for leaks to fully dry), take a dry paper towel to all the cooling hoses and connections you can reach, at/under the reservoir, from the rad to the block, above/below/beside the water pump (esp. above between the pump and the head) the return line at the back of the pump under the exhaust, the heater hoses on the back and right up to the firewall. Start by carefully inspecting into the water pump bearing weep hole (if there is one, either on top or below) with a good light for signs of green fluid. Even then it may still be leaking and evaporating, so you won't catch a drip, but still worth the effort if you can confirm where the problem is.

I'm rather guessing there's a good chance it's just the cooling system and not the head gasket based on what you've said so far.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now