Look. This is an Arduino forum. Everything here is an approximation, even the the species of some of the visitors here 
But back to the topic - yes, it sounds to me like something else in your mains system is injecting some voltage into the earth connection and that earth connection isn't linked to ground as well as it should be. It's anyone's guess what it might be that's injecting that voltage. As you still get the voltage with the iron off it is unlikely to be that causing it - that's just the point where you have access to the earth connection, so it's where you notice it.
I would recommend getting someone in to check out your earth connections properly.