Coolant reservoir cap

So the level keeps dropping in the reservoir? If you have to keep adding coolant to the reservoir, the system is leaking or losing coolant somewhere. Have the system pressure tested to find the leak. Is the reservoir itself leaking? When the level gets low enough, first the reservoir will go dry, then the radiator level will start to drop.
Here's how the reservoir works: coolant is under pressure from the radiator cap seal. As coolant heats up from hot engine, it expands, and pressure forces the radiator cap seal up, coolant is forced into the reservoir-remember the hot mark on the reservoir? When car is shut off, coolant contracts, creating a vacuum in top of radiator. This vacuum works to suction coolant back from reservoir into the radiator, keeping the radiator topped up, and coolant should now be at the lower cold mark on the reservoir. It is a closed, sealed system. The only loss of coolant will be a very small amount of evaporation from the reservoir. If coolant level keeps dropping, there is a leak somewhere.

Has coolant level been going down in return reservoir at all, as in have you had to add coolant recently to bring it up to top level? Have you tried running engine from cold, with heater on and let it run up to normal temperature with cap off return reservoir? If not do so, and watch level in reservoir and if it goes down, add as needed until engine running at normal, and not overheating. Allow rad fan to kick on & off a few times before re-installing cap on reservoir. Pay close attention to engine temp while doing this. If it goes high, shut it off and let cool. Be careful, my son's Passat just had overheat problem and turned out to be impeller on water pump ( internally) was spinning on shaft of pump. Thermostat is certainly a consideration if coolant is full, and all air bled out. Keep me posted on what happens when you run with cap off and keep reservoir full. Don't forget, heater on while doing this.

WAIT UNTIL YOUR ENGINE IS COLD,YOUR RADIATOR PRESSURE CAP IS LOCATED ON THE COOLANT OVER FLOW RESERVOIR, THATS WHERE YOU ADD COOLANT WHEN COOLANT LOW,YOU POUR COOLANT IN THE OVER FLOW RESERVOIR UNTIL COOLANT LEVEL REMAIN AT FULL COLD MARK DONT OVER FILL OVER FLOW RESERVOIR, WHEN COOLANT LEVEL STOP DROPPING WHEN ADDING COOLANT AND COOLANT LEVEL STAY AT COLD MARK ON THE RESERVOIR YOU RADIATOR HAS THE CORRECT COOLANT LEVEL IF COOLANT LEVEL KEEP DROPPING CHECK FOR COOLANT LEAKS AROUND RADIATOR AND THE TOP AND BOTTOM RADIATOR HOSES, LOOK FOR COOLANT LEAK AT WATER PUMP GASKET OR SEAL OR LEAK AT WATER PUMP WEEP HOLE IF YES REPLACE WATER PUMP.

You probably have a slight leak from the coolant filler cap and you have lost coolant by evaporation over the four months. Check the seal on the cap and replace it or the cap.

Check ur radiator cap! Mostly when the engines become hot, coolant goes thru the coolant reservoir and comes back to the radiator when it cools down. Damage radiator caplet the coolant goes thru the reservoir but can't siphon back the coolant to radiator , thus always overflows the reservoir and the radiator dries out from its coolant! Check also if there is LEAKS in the bypass hoses and CAP PLUGS! Hope that might help u out!

The radiator is that flat panel shaped object in the front-most center of the engine compartment. You don't add coolant and water directly into the radiator. On the right side of the engine compartment is a plastic overflow reservoir with a blue cap. That's where the coolant/water mix goes. Run the engine while adding coolant and water. There are fill marks on the reservoir indicating how much coolant/water mix is required. With the engine running, if the radiator is very low on coolant it will repeatedly draw fluid from the reservoir. Continue to add coolant/water mix until the situation stabilizes and the reservoir is filled to the high fill mark.

fill the reservoir with the coolant. it is a GOOD thing you dont see steam or a leak I hope.YOU DO WANT TO CATCH THE PROBLEM BEFORE IT REALLY OVERHEATS OR YOU MAY HAVE MORE COSTLY PROBLEMS. fill the reservoir. BUT, it may only fill the "radiator" once you do this as it cools off. As the radiator water cools it sucks the reservoir water into the radiator. as it heats up and the water swells (increases in volume) it pushes the excess into the reservoir. when the engine is good and hot and you dont have any coolant in the reservoir, that is a sign that the radiator is low or empty also.
when the engine is cold, find the radiator cap and remove it, add as much coolant you can there. fill the reservoir also. If it has been several years since the radiator cap has been replaced, I would do that now ( it is a cheap investment). if the gasket on the cap is not sealing well anymore or the exspansion valve mechanism on the cap is not working properly anymore, then it will let coolant pump out into the reservoir and not hold radiator pressure. if the reservoir is empty the radiator itself is probably too low or empty.

Twice in two years now my Saab 9-5 has had a pin-hole in the oil cooler, which has caused oil to flow into the reservoir. A way to tell is if coolant/oil is coming out of the reservoir cap. The reservoir cap is rated for the pressure of the coolant system, which is significantly lower than the oil pressure. When there is a hole in the oil cooler, oil will be forced out at relatively high pressures, causing the reservoir to fill and then over pressurize and spill out of the cap.

this was caused by the cap being gone, there is zero doubt of this, I hope u have not damaged the cylinder head gaskets, time will tell.

Your description may actually be your answer, so to speak. You mention 'generally as engine is cooling or if coolant is added too quickly'. It sounds to me like the reservoir is over-filled.
When the engine is cooling, coolant tends to flow back to the reservoir (plastic 'jug' where you add coolant). A bad radiator cap or one that was replaced with too low a PSI rating can easily allow too much to fill the reservoir and and occasionally overflow it. Most reservoirs use a rubber tube to drain excess coolant so it may appear to 'leak' near the front left wheel or left side of engine.
Replace your radiator cap. (did they pressure test the cap too?)
Make sure you check your owner's manual for the proper PSI. My sources show 15-16psi, but that sounds a little high to me. Also, make sure you have the correct (50/50) anti-freeze/water mixture.
After you replace the cap, make sure the reservoir has the right amount of coolant and isn't over-filled. (To 'fill cold' line) Drive the engine until hot, then park the truck and see if the problem is fixed. Good Luck!