I found a pen in the car that I rented? Whose is it?
You can keep the pen. We can assume that the owner gave up hope (מתייאש) and if you are living in the diaspasra we can also assume that the owner of the pen was not Jewish. It does not belong to the rental company.
Bova Metziah 26b, Rambam Gezelah Veavedah16:4, CH:M 259:3, 262:5, 266:1