That and their propensity to fall out of the outlet at the slightest provocation.

https://chemistry.stackexchange.com/questions/54915/why-is-o2-oxygen-colorless-and-odorless-while-o3-ozone-is-a-pale-blue-gas-wi

It's the smell of ozone and spluttered metal vapor. When you plug and unplug, it creates momentry sparking, which generates ozone and vaporises a tiny amount of the pin/contact metal, that can create the faint smell. It's normal so you should not worry. Unless you can see pronounced flashing and arcing when unplugging etc, then it's not a problem.