I live in a house converted into a shop and two flats. The flats are on the second and third floors and accessible by the same staircase. On the staircase I found a tenner. I know it can't be my flatmates as he is away. I don't know if it's mine or isn't mine and I don't know if it belongs to the guy downstairs or doesn't belong to the guy downstairs.

I don't want to take his tenner but don't want to leave my tenner for him to pick up. I did try to knock on his door and ask if it was his (not necessary expecting an honest answer) but he was out.

Do I

a) Keep it

b) Replace it on the stairs

c) Give it to charity?

d) Stop worrying and learn to be a mature adult who makes his own decisions without consulting the internet?