Here would be my procedure following that info:

Id have a look under the drivers side behind the wheel where the timing cover is and see if there is any green staining. If it is dry and clean, then the water pump isnt leaking.

The second would be checking under the intake manifold (both sides as the thermostat housing seal can fail) where the water transfer pipe lives in the V of the motor for leaks in there. The heat of the engine could burn it off, though you would absolutely smell it.

Following that I would be pulling the spark plugs and seeing if they are rusty, chalky or fouled then you know the car is consuming coolant.

Also, if the way you are checking if it is leaking is the overflow bottle, overflow bottles are very old now and perish. Check its not stuffed.