Is it rude to borrow an unused power outlet without asking?
Sometimes I'm at the doctor's office, at the library, or even at the grocery store and see an unused power outlet. My phone is dying. I feel weird plugging in, but I feel even weirder asking for permission.
If it's a public place and you have legitimate business starting there for a while, I don't see any issue with it. Eg. Doctors waiting room and you have an appointment, restaurant and you're dining there, etc.
Some places you're not supposed to plug anything in unless it's tested and tagged. Unlikely to be an issue in a public area, but if you're somewhere that the power getting tripped would be a safety issue then best to ask first.