Q: Coolant leaked out of coolant reservoir

Leaks normally will leave a white residue or wet spot at the leak source. If the leak is not obvious a cooling system pressure test may be necessary to determine if there is a leak present. Another possibility could be a air bubble got trapped in the system while it was being filled and the level dropped once it dissipated. One thing to note is that the cooling system should never be opened when the engine is hot or the system is pressurized. Doing so could cause serious injury. You may want to enlist the help of a mechanic, such as one from YourMechanic, who will have the equipment necessary to locate any leaks and repair the engine cooling system.