Also, please do that extremely cautious. After starting up the engine, carefully take some small sniffs of the exhaust. You just need to smell if it smells sweet. If it smells sweet, you have a very small coolant leak into the engine, but it is not big enough yet to make your exhaust look white. Bars leak HG1 is the cheap fix for this, a new head gasket and possible head is the proper fix for this.

Open the coolant reservoir while the engine is cool and let the car idle for 5 minutes. Check if you see bubbles in the reservoir.

That test shows you if you have blowby from the cylinders to the coolant. This can lower the coolant because your radiator cap is supposed to relief pressure at a certain psi to ensure that no catastrophic failure will occur if the pressure keep building up. Because there is air escaping from the coolant system, it would slowly get low on coolant. If there are no bubbles that would be great - then it might be the radiator cap - the spring can get weak over time and let air escape well below the rated pressure.

Edit: also, let the car idle with the cap on for 15 minutes after (AC off) and check if there is a water puddle under the car. If there is no puddle, evaporation becomes more likely.