If UPS unsuccessfully tried to deliver your order, the tracking link will indicate this.
In this case you will receive a notification with instructions in your mailbox. UPS will try to deliver your order again the day after. They will not automatically leave the package with the neighbors. If UPS does not succeed the 2nd delivery time, it will be send to a UPS depot, where you can pick up your delivery.
In case your order was shipped with DHL Global Mail, they will try to re-deliver your package two times. After that your order will be returned to Fatboy.