Hot water cylinder

[daisyfraser]

Am in new (5 months) home and the old Baxi boiler has been fine until today. It seems to be permanently heating the water despite everything being switched off and the boiler not firing. The water is still hot and I can hear a constant humming from the cylinder.
Is this something that can wait til Monday, does anyone know?
Bit anxious about it.
Thanks all

Possibilities:
Could there be an auxiliary electric immersion heater?
Or is the 'cylinder' actually a heat exchanger (a reservoir of extremely hot water that heats water flowing through it when you run a tap).

So kind of you to respond. Shortly after I posted - really very worried - I manually switched on the hot water at the control panel and it fired up. After three minutes I switched it off. The water-heating immediately stopped.
So somehow the burner must have stayed alight after the timed event earlier in the day. No idea why it would do this and burn gas all day. Thought I'd report the outcome for others as it may be of help.
I was planning to replace at expense. But this is probably a timely signpost.
Thanks again, Errol