Finding Water - Dowsing

[Mortblanc]

If you start pulling up carpets or new floor coverings in the designated room you will find some kind of access door. All cisterns have access for checking water levels and doing repair work. I have never seen one that could not be accessed for checking water level.

Mine has a locking man hole cover.

There should also be an overflow outlet for excess runoff. Now that you know what you are looking for they should be obvious if they have not been pulled out or covered by new construction.