Sorry for the lengthy delay in response. I didn't have notifications enabled until just now, and didn't think to come back and check this thread.
My method to measure in my OP was to remove the ground lead from the probe (not the scope power cord) and then measured directly. I know that the outlets are wired correctly, and i have a plug and short wire length with the correct indicators for hot, neutral, and ground. The scope was set to use AC as the trigger.
I am aware of the risks, and was comfortable with them.
How is the scope connected to the mains? Direct, voltage divider, transformer?
3rd, 5th and 7th harmonics are common. 3rd would be mostly generated locally, typically an unloaded transformer will generate some 3rd due to hysteresis. However, even though industrial loads and distribution systems do generate a lot of 3rd, you don't see them much because in a 3-phase system, they cancel out.
5th is generated by induction motors and certain generator configurations.
What you have looks like an unloaded transformer to me, although rectifier/capacitor systems could be contributing as well. THD is supposed to be under 5% and it looks like you may be approaching that.
B/T/W, that is a nice clean FFT, especially as a freebie add-on to an entry level scope. What exact settings (window type, etc) did you use?
Thanks for that feedback. I will have to redo the measurement re-check the settings I was using on the FFT - this was over a month ago so I don't remember them. I also remember doing a fair amount of fiddling with the settings trying to make sense of it.
I might have to go on a search for unloaded supplies. Would this be only for linear supplies (both the AC/DC and AC stepdown), and would unloaded SMPS be included too? I am sure that there are a TON of scattered SMPS (wall warts, phone chargers, etc), but probably not as many linear supplies, and even fewer purely stepdown transformers. The only stepdown transformer I am aware of would be for the furnace/thermostat - and maybe the doorbell.
There are probably enough options to just shut off breakers until it is eliminated (assuming the source is in the house).
EDIT: I now realize that maybe the question about FFT settings was directed at Radiolistener (I would like to know how he got that plot as well, and what scope he used).