Glow plug question on a VW Passat 2002

[sternal]

Have recently purchased a 2002 VW Passat 130 tdi which on all accounts is great.
I have noticed that the glow plug light goes out almost instantly which is normal but on occasion the GP light stays on for up to 10 seconds (relay is set at 10 seconds) will stay on even if the engine is warm then go off before or at 10 seconds.
Been told not to worry ,all the same would like to know why the light goes off instantly at times and at other takes a upto 10 seconds !

Any thoughts on this ?

[Timburrows]

It's probably the 4pin temp sender in the water outlet pipe rear of cylinder head could just be corrosion on wiring terminals easy to change it pushes in and a clip slides across to hold it in place

[sternal]

I thought it would be a sensor, will see how it goes.

Worth worrying about ?

Thank you for help

[dave friday]

Some fuel filters have a water sensor that uses the glow plug light to indicate that some water is in the bottom of the filter...
Perhaps?