This got me thinking...
My scope is plugged in to a surge protected power board/extension lead. Those things have caps and MOVs and other bits and pieces in them...
Its very subtle, and you have to look close, but it seems the neon which indicates "surge protection active" is lit. From a distance it doesnt look like anything, maybe just some light reflecting from the glass bulb of the neon through the coloured window, but upon closer inspection it is definitely putting out some light.
I moved my scope to another power board which doesnt have this neon lit, and Im not seeing the same thing. Meavuring voltage between USB ground and scope ground I get millivolts if anything. So maybe the surge protection of this other one is busted or leaking or something.
I could buy a new one, or I could try to fix this one I guess, I'll just have to buy some parts as I dont tend to work with higher voltages like mains.