Owner info
how can I locate an owner of an air tag if only the serial number shows up
It is up to the owner of the AirTag what information they want to provide if any. You will get the serial number and the last 4 digits of phone number if you wanted to verify that information. There is no way to determine the owner other than the information they provide. Note that as long as your phone has Bluetooth on or any other phone that has Bluetooth on within 30 feet, the owner will be able to see its location and possibly track you. This support article not only has information about AirTags, but also how to disable it by removing the battery to stop tracking.
My recommendation would be to just leave it at a location that the owner would be able to find it after they have determined it has been lost. This could be in a semi-hidden location as the owner could use precise tracking and audible alerts to make finding it easier.