As the engine coolant heats up it expands and flows to the overflow. When it cools it sucks it back from the overflow. Over time the overflow can get near empty, lots of time and I suspect how humid the air is.

If the coolant gets too hot it will put too much coolant in the overflow and it will empty on the ground while driving, then again the remaining overflow contents gets sucked back into the cooling system.

It could be that your cooling system is dumping while your driving or sitting still in like at Jack in the Box.

New radiator cap is an excellent idea. How hot is the coolant temp getting during driving?