Literally the first link from searching "luigi McDonald's"
Authorities said a customer in the restaurant thought he matched the description of the suspect in Thompson's killing and notified an employee, who called 911.
https://www.newsweek.com/mcdonalds-worker-luigi-mangione-private-security-1999217
That customer was almost certainly law enforcement, they just needed a scapegoat to call in the tip because they couldn't say how they found him.
The McDonald's worker had no idea, that's why their call was basically: "some guy said another guy is dangerous and wanted by police, I don't know who".
And cops immediately swarmed and singled out Luigi.
You may believe you're "reasonably tuned in" but this information was out within like 24 hrs of his arrest everywhere.