Lindsay lost the phone in New York City, as she left the V magazine party at the Boom Boom Room. We're told our Good Samaritan, who worked next door to the venue, spotted the lost phone and took it home.

According to our sources, the screen read "LINDSAY LOHAN'S PHONE" when he plugged it into his computer. He somehow figured out how to contact Lindsay's people and then returned it ... without even asking for a reward. And you guessed it ... she didn't give him one.

We're told Lindsay was ecstatic ... the phone contained lots of important phone numbers numbers and emails, as well as personal photos and "a lot of stuff" that she wouldn't want published.