Prerequisites: Max out Dev's level 2 friendship
Reward: 250 G
Unlocks: Dev level 3 friendship
Dev the mailman could use a friend, and you're just the person to do the job. To further befriend Dev you'll need to continue speaking to him at his workplace in Town Hall. Dev is found there most of the day, making it rather easy to deepen your relationship with him. That said, don't forget that Dev turns into Dan partway through the day. Dev and Dan have distinct relationship bars, and boosting one will not affect the other. Poke your head into the mail office each morning and meeting Dev shouldn't be too difficult.
Max out the second level of friendship with Dev and he'll post a notice on the Bulletin Boards around town. He has misplaced his brown suitcase, and needs help tracking the luggage down. You can meet with Dev between 12:00 to 20:00, Monday to Sunday, to discuss the problem. Dev lives in the south of Moonbury, and given the amount of pirate gear decorating his home it's difficult to miss.
Dev is freaking out about his missing briefcase when you enter his home. He pleads for your help, then notes that the last place he had it was at Moonbury Cliff, to the north of the Church. Check the area and you'll find a hole in the ground, in front of the cliff's lone tree. Inspect the hole to find Dev's Briefcase, as well as trigger a conversation with Dan, Dev's alter-ego. Looks like you found your culprit.
Take the briefcase to Dev. He'll accept its return - and the reasons for Dan's prank - as a lesson learned, and your relationship with him will deepen to the third level.