Plug in incense & scented oils

[Sam Leon]

What are your thoughts on plug in scented oils?

Personally I do not have a problem with it, until I have a tenant that went a bit overboard with it and put it in every room, sometimes different scents on different walls of the same room.

Then you end up with them plugged in behind a sofa, or against a shelfing unit where you no longer see them.

Then sometimes the oils run dry and she forgot to refill, for days.  When that happens, the plug gets hot and the receptacle and the wall directly above it get black sticky smudges, like these.

Can a plug in scent device, when running out of oil for days, still burning to the point of staining the receptacles and walls, pose a safety risk?  Can it overheat the receptacles and connected wiring?