noun
notice of absence; mail hold request
A form submitted to the post office to request that mail be held during a long absence, such as a trip.
長期旅行の前に、郵便局に不在届けを出した。
Before a long trip, I submitted a notice of absence to the post office.
noun
missed-delivery notice
A notice left by a delivery service when a package could not be delivered because the recipient was absent.
帰宅すると、ドアに宅配便の不在届けが貼ってあった。
When I got home, a missed-delivery notice from the courier was stuck on my door.
Compound of 不在 (absence) and 届け (notification, report).