So what was the outlet there for? If it's on a public building but not meant for public use, it should have been secured, either by locking it or having it shut off inside the building. Actually, the drinking fountain comment is a good point. Obviously, a drinking fountain is there for public use. But what if it's just a faucet? Is getting a drink from a drinking fountain okay, but not a faucet? Is charging a phone okay, but not a car? Where is the line here?
Exactly where the company chooses to draw it, in most production companies taking one chocolate off the production line and with you home is a firing offense even if it's worth five cents. Things that are provided for work (materials, tools, services, products, whatever) are there to let everyone do their job, any other incidental use you might want it for is up to their acceptable use policy. Would a network manager accept that people connected their own devices to the internal networks to siphon off a few bytes of the Internet connection to check their mail? I very much doubt that unless you work in a BYOD workplace or have guest networks set up specifically for that purpose.
Most employers tend to take a reasonable approach on marginal costs (browsing the Internet, private call on work phone, printing two pages or making five photocopies, charge your cell phone) because being an ass works both ways but strictly speaking they could put me in a secure compartmentalized zone and deny me bringing almost anything in and out except myself and the clothes on my back. Of course then I'd say I'm working for paranoid nuts and not Top Secret military systems and find myself a sane employer, but it'd be totally legal. But I have worked on systems that simply didn't have Internet access, go to special terminals if you need it.
So how far could you go in the absence of any written policy, oral approval, signs or any other obvious indications? Well there's implied permission, if they offered parking spaces and those parking spaces had EV chargers on them (like one per space) I'd take permission to park to also imply permission to use the chargers. But if there just happens to be a socket in the parking lot so anyone working there could run a power tool, I'd say you don't have that. It could be things that are so commonplace that you wouldn't ask, like using the bathroom if you already have legitimate business in the building. Charging your car isn't that though.
I think they're technically correct, though I'm a repeat offender of "accidental theft of ballpoint pen" and if the law was applied to the fullest I'd have way more than three strikes. I think it's mostly because siphoning off gas to power your car is generally recognized as a crime, siphoning off electricity to do the same sounds equivalent. It doesn't sound like something you could do without at least some form of permission. It's all fairly relative though, if EVs become common it might be commonly understood that sockets are there for charging them and you'd need to explicitly deny it. But that's ten, twenty years from now and not today.