After FedEx or UPS tries to attempt the delivery of a package for the 3rd time and you're not there to sign for it or you don't leave a signature on the door tag stating they can leave the package without you there then the UPS or FedEx driver will return the package to the sorting facility and then it will be sent back to the shipper of the package.
Both FedEx and UPS have always left a door tag on my door when I wasn't there to sign for the package and all I had to do was leave my signature on the door tag stating they could leave the package and that signature I signed on the door tag was good enough for a signed package and they left the package.
So if you can't be home and they leave a door tag then sign the door tag and they should leave the package at your door or other place of your home.