Plumbing : My shower works even when mains water is off

[Sheaintheavyshesmymother]

I turned my mains water off before I left for work. When I got back I had a very unusually long shower (electric) of 25 minutes. After the shower Imtried to brush my teeth and the taps were dry which reminded me the water was turned off.

what on earth is going on? This is definitely the main water supply stop cock. I am so confused.

if it’s relevant I live in a 1920s flat, second floor of three.

you must have a storage tank up in the attic.

You probably have a hot water tank which you have now emptied...

but i am in a flat. there's no attic.

But do you have a hot water tank? Maybe in an airing cupboard?

Another option is that there's a different stop cock for that supply, possibly something to do with it being a flat. It may be it a basement or communal space.

i literally cannot for the life of me imagine where a tank could be with the layout of my flat (small, 1 bed). But I guess yes it must be a second stop cock! But hat seems so mad as this is the stop cock that turns off the kitchen and the toilet… **

Is your flat a purpose built building or a converted house?

If you've an electric shower it will heat the water, it's probably a cold water tank somewhere, not a hot water cylinder.
They're usually in the loft (have to be somewhere higher than the shower, it's fed by gravity)

Please describe or photograph this electric shower.