It's a check for $2 million, issued to a man with a California driver's license. And it was found in a subway car in Madrid, police say.
"We have his name. We're trying to locate this person," a National Police spokesman in Madrid told CNN on Thursday.
"We won't just hand it over. We have to verify that the origin of the money is not illicit," said the spokesman, who by custom is not identified.
The check was issued by Bank of America, and it's post-dated to January 2014, the spokesman said.
A maintenance worker for Madrid's Metro, or subway system, found it Wednesday. A train running on line 5, which roughly traverses the capital from east to west, reported a problem with doors that wouldn't fully close at the Diego de Leon station.
The train was sent to a maintenance yard, following protocol, and a worker who inspected the doors found a brown leather wallet that contained the $2 million check, the California driver's license and some credit cards, a Metro de Madrid spokeswoman said.
"It appears that the wallet was stolen on the Metro," the spokeswoman said. "The thief looked inside, grabbed the cash -- but didn't see the check -- and then threw the wallet away."
Usually, these kinds of discarded stolen wallets end up on a station platform. But this one landed in the doorway of a Metro car, preventing it from closing properly, she said.
Madrid's Metro handles an average of 2 million passengers a day, but this is believed to be the first time that one of them was carrying a $2 million check, the police spokesman said.
Why would someone with such a large check be riding in the subway, instead of in a chauffeured car?
Police would like to ask the owner of the check that question, too, the spokesman said.
Police have contacted the U.S. Embassy in Madrid for help in finding the check's owner, but the embassy had no immediate comment.