Nah, vast majority aren't locked up here, from experience. Pubs in the UK tend to have them, as do larger shops, community halls, libraries, I've seen old red phone booths repurposed as AED storage, in Birmingham city centre I saw a really modern one like one of those light-up ad screen / phone booth / touchscreen signs, which could simultaneously call emergency services, deliver CPR advice on the screen and dispatch the AED stored underneath - was awesome. Honestly I'm surprised (and pleased) that they don't get stolen/vandalised more considering the amount that are just out there - though I don't know what you'd do with a black market AED lol.

I have yet to see one that's properly locked up [that didn't have easy to break emergency glass, anyway]. My workplace has a few in special AED boxes, but those have the emergency glass and the hammer.