What could be causing my electric hob to leak voltage?
You say the hob is old so I would get an estimate to replace all 4 or 5 controls (the others may not be too far behind the one in question in wear and tear) along with an assessment of the hob's overall condition and compare that against buying a new one.

My Mom has a ceramic cooktop and I'm going to take a meter with me when I see her later today. Now I'm curious.