Why do I see a 20 Volts peak-to-peak sinudoidal signal @ 50 Hz on my ESD mat cover?
Static Dissipating mats are rated in units of GigaOhms per square and not 1M Ohm like wrist straps.  

So you would get likely more voltage on your 10:1 probe by touching the tip with your 50~100pF finger tip using your body as an E field collector.  The impedance of such mats is not a simple ground but a static dissipating insulator.

If you doubt the suitability of your mat, do some surge current test with a 50 ohm measurement method to measure xx uV current rise time and amplitude across an interface 50Ohm shunt to coax to DSO.

I can dig up test methods, but the concept of protect is a matter of voltage and Z(f) impedance.  I can dig up test methods if concerned. ANSI/ESD S4.1 "Worksurfaces - Resistance Measurements".