The victim left a $700 wallet, containing $1,200 in cash, on a table at Shake Shack in Westbury on Saturday, Nassau County police said.
Now, police are trying to find another woman who they said sat at the same table, took the wallet and cash, put it in her bag -- and failed to return it or turn it in.
The wallet also contained paperwork, police said.
Police identified the victim only as a 23-year-old woman.
Police said the theft occurred just before 8 p.m. at the Shake Shack on Old Country Road. The victim was at an outside table -- and left the wallet. Police said the suspect then sat at the table with a friend, took the wallet, and later left.
When the suspect ordered food, police said, she gave her name as "Amanda."
Police are asking anyone with information to call Crime Stoppers at 800-244-TIPS. All calls will remain confidential.