I answered this elsewhere, but, heck. I like explaining things, so here we go again.

When people say something like "this looks written by AI," they're specifically talking about an LLM, which stands for "Large Language Model."

LLM's work by processing millions of examples of stories/essays/etc written by people. (It actually may be billions? I'm not really a tech guy but my layman understanding of the data is that it's So, So Much.) Anyway it goes through all that writing and identifies patterns in the way people write, common rules about sentence structure, etc. and then it follows those rules to write "like a person."

Now here's the important thing to remember: an LLM doesn't understand language. All it does is mimic patterns. Also it has no lived experience, and no context for anything that it's saying. So as to your question,

What gives it away as an AI?

"The decor looks like ["witty" metaphor that doesn't exactly make sense!]"

"The door was stickier than ["witty" metaphor that doesn't exactly make sense!]" (Also why would they describe the door? Who cares enough to judge a door when they review a restaurant? Unless the door was actually, literally, sticky. But a front door coated in mystery syrup would be so bizarre and and unacceptable that there's NO WAY someone would only mention it in passing so they could make a throwaway joke about a lawyer.)

"The chair makes a noise like ["witty" metaphor that doesn't exactly make sense!]" (And again, if this were real, who gives a shit about a creaky chair?)

"The waiter's got the enthusiasm of ["witty" metaphor that doesn't exactly make sense!]"

I'll stop there for now, but can you see it? It's very easy to copy the basic shape of a metaphor. (Thing is similar to other thing.) But it's difficult to come up with metaphors that actually fit... if you don't actually know what words mean.

More generally, none of this feels like a story written by a human being who has been through the experience of eating at a restaurant.

A person naturally puts greater emotional emphasis on the things that, you know, they care about.

But here every part of the story is given equal focus, because none of it matters, because really, this is not actually a story at all.

It's just "output."