radiator is hot when turned off.

[tjslash]

I put heating on for first time this year last night and my kitchen one heated up even though it is always turned off and has never heated up before.
The thermostat knob is on 0. Why has it suddenly turned itself on?

 

[JohnD]

the thermostatic valve has failed "on".

If it is a common modern type, you can buy a new, identical one and change the heads.

Or possibly someone who spends a lot of time in the kitchen getting cold has got fed up with your meanness and adjusted it.