Location: Desert Near Alubarna
Petitioner: Officer Segem
Reward: Family Ring, 18,000 Berries
Objective Reward: Perfected Exhaust Ball

In order to trigger this Side Story you need to first complete the main story events of Alabasta, which means defeating Crocodile and leaving Alabasta altogether. The main story will chuck you back into the region shortly after your return to Waford, at which point you can find the NPC responsible for Battlefield Courier. He's at the Royal Army Garrison on the east side of the Desert Near Alubarna, just south of Alubarna itself. 

Segem is standing along the east side of the camp, and he's concerned about his son Hator, who joined the rebels during the recent civil war. He doesn't know where Hator is, and now that the civil war is over Segem wants a status update from his estranged son. Segem will give you an Officer's Letter and tell you to head to the Rebel Army Camp, on the west side of the Desert Near Alubarna.

Trek over to the Rebel Army Camp. There's a campsite right in the middle of the base, and if you look a little ways north you'll find a largeish man sitting beside a ladder. He doesn't know Hator, but suggests you check with the guy at the top of the ladder. Climb and walk a ways south to find said guy, who knows Hator. He points you to the canopy on the ground level, just east of the campsite.

This last guy, Hator's Partner, will tell you that Hotor left to check out the Baroque Works' Hideout. There are two entrances to this place, and the secret entrance in the south of the Royal Army Garrison is the faster route inside. Jump to the Yoisa Sign in the south of the desert, head into the Royal Army Garrison, and go down the ladder into the hideout. You'll find it just south of the garrison's campsite.

Hator is standing in the hideout, two ladders down and by a table. He'll prove a bit standoffish at first, but Nami will shame Hator into writing a Son's Letter. Take this to Segem to sorta resolve their issues, as well as complete the Side Story.