I accidentally stuck my oscope probe into my ESD mat ontop of my work bench. I happened to look at my oscope. Surprise! There's a 20Vpp 60Hz voltage on the mat! (see picture) (note, the ground clip of the probe is not connected to anything)
The ESD mat is not currently grounded (before anyone jumps on me, I had it disconnected to move it, and haven't put it back yet because I haven't been dealing with anything that is ESD sensitive.) When I connect the mat directly to earth the voltage flat lines and all is well. When I connect the ESD wrist strap to the mat and then to earth, the voltage reduces to about 3 or 4Vpp. The wooden bench top itself measures nothing at all. There is a power strip screwed into the top of the bench, but its casing is ground to ?earth.
My question is how is that 20Vpp on the mat itself? If it isn't actually there, what am I witnessing? Should I directly earth my mat or is that dangerous? I have stopped working on the bench until I figure out what is going on.