A very reliable 10 year old Honda Shuttle started to loose a lot of water from the radiator.
Thames Ditton Trident Honda diagnosed & changed the radiator for over £400. Even though I knew there was no leak in the radiator it did look like it had sealed itself (perhaps with limescale or dirt) as there were indicative anti-freeze stains.
The leak continued & I could not trace it.
Finally I reverted to using K-Seal, exactly, as instructed (otherwise you can block your radiator rendering it useless) and again no leak fix, but the copper particles in the K-Seal clearly indicated the true source of the water leak.
Water was being pushed past the expansion tank cap thread and being lost.
Some PTFE tape sealed the cap for a few pennies. Use Gas rated PTFE tape which can withstand the temperatures that the expansion tank is exposed to.