If it was found outside and you have no idea who might have dropped it, it's yours.
If you found in inside of your workplace not open to the public, I think you have an obligation of trying to find the owner.
I once found $20 at work right outside of the ladies room door. I put a note on the bulletin board saying if anyone lost money, come see me. About ten minutes later a coworker came up to me and said she went to the ladies room on her way to lunch with $20 in her pocket. When she got lunch at the work cafe, the money was gone and she couldn't pay for her meal.
The $20 bill had some writing on it and she identified what it said. I was happy to hand it over to her.
I occasionally find a bill or two floating around. My DH found a $50 bill in the street blowing around once. I always feel bad for the owner, but there is no way to find out who lost it.
Now if it is a large sum....you should call the police.