Hold you for 24 hours to see if anyone reports a crime and describes you as the perpetrator.
When no one does, find a crime which seems plausible for you, and where they’ve gotten a description that could possibly fit you.
Interrogate you about it, giving you your lawyer of course. Assuming you do not have a solid alibi for that particular crime, there’s a real chance you’ll be charged and eventually convicted.
If you do have a solid alibi, they might keep looking for other crimes to charge you with, or they might give up.
If they give up, they’re likely to charge you with something related to wasting their time, for which you will at minimum have to pay a fine.