60Hz 20vac on bench top
You are seeing stray voltage being picked up by an isolated (floating) surface.  You will see something similar if you touch the tip of the probe while wearing insulated shoes, or attach a long cliplead to the probe.  The mat, your body, the cliplead, all act as antennas.  There are 60Hz electromagnetic fields in your lab, created by the wiring in the walls, the power lines, etc.  Since the scope probe has a very high impedance, it needs only microamps of current to display a signal.

This is normal, and not dangerous.  I would go ahead and ground the pad.