Dont laugh at this but it might help you locate the leak, hear me out.
I assume there are no wet patches left over night on the ground under the engine area, if not then I would stick a towel, large sheet, blanket or bit of cardboard under the car, start the car & leave it to be build up pressure in the system for anything upto an hour to give it time & see if it drips anything down onto the material below then this would give you a general idea of where the leak is as it must be going downwards obviously, then you can start tracing threw.
If there arent any overnight then sounds like a tiny pressure leak as it would seem to only lose water when the system is running & pressure is maybe making the minute split in a pipe that usually when not in use would look like a completely sealed pipe, release droplets of water cause twice in 3 weeks it would say its losing water @ a very very slow rate.
Have you checked pipes around the areas you can, squeeze them to see if it releases any water from anywhere, even the smallest of split pipe would be causing your problem, try this when engine is running so you are adding little pressure to the pipes, of course with care.
Have you had the cambelt done, when its done they should also replace the water pump always, as its a 98 the pipes are fairly old so might have a split & sounds very likely TBH.
