You've received some good advice so far, and some not so good.
Adding anything to a cooling system, like a "car-repair-in-a-bottle" is a really bad idea, especially at this point. It is way too early to add any crud to your cooling system. Adding that stuff should be a desperate, last chance, Hail Mary pass, kind of a deal. Diagnose the problem correctly first.
Coolant that is going "missing" is typically not that hard to diagnose. Any good, experienced mechanic, in a good shop, will be able to figure it out. Dyes and all that are unnecessary in virtually every case.
The idea that a leaking head gasket always leaks into the oil is just flat out wrong. I cannot count the number of bad head gaskets I've replaced over the years and I'd say 25% or less had coolant in the oil, if that.
It is entirely possible that coolant is leaking into a combustion chamber and being burned and it is not enough to be seen out of the tailpipe, yet. It may be that having someone follow you, who knows what to look for, on the highway, will be helpful.
Pressure testing doesn't have to only be done with the engine off. Sometimes to find a leak it helps to have the engine running while adding pressure to the system with the pressure tester. Don't exceed the maximum pressure, though.
Pressure testing should be done with both the engine cold and at operating temperature. Hot engine parts expand, cold parts contract and that movement may be part of what's happening.
Looking at the spark plugs will tell you a lot. Cylinders burning coolant usually have plugs that are cleaner looking or the insulators may have a slight color cast similar to the color of the coolant. Either way they will look different than the others.
Other good possibilities are a leaking side tank or a hose. Gently wiggling hoses and such while under pressure may reveal the leak.
It may be the heater core that is leaking. The box that houses the heater core may be wet or dripping coolant. Typically you can smell the coolant, or the windshield may fog up, when the heater core is bad, but not always right away. It may take a week or two for symptoms to be noticeable.
You can do a lot of diagnosis yourself with a good inspection mirror, a bright, light source, some time spent looking around thoroughly and some patience. Drive the vehicle for a bit to get the coolant hot and the cooling system pressurized and have at it. Check all around the water pump area, top, bottom and sides of every coolant hose, the radiator tubes and tanks both front side and back side, all around the engine top, bottom and sides, and then check it all again. Engine hot, engine cold.
Finally, cooling systems may "use" a little bit of coolant over time. Particularly an older vehicle. There doesn't have to be any kind calamity for that to happen. Wait another week and see if any more coolant needs to be added before doing anything else.
Is the side tank part of the pressurized cooling system or not ?
