This is a quest where you go to the sector all the way to the south-east in Lucas' fortified island (Sector 23). It should be on the second of the small island in the northeast of the sector. The longer you put off this quest, the more natives will quit.
Infiltrating Sector 23 Edit
Capturing this sector is definitely not easy. If you go by the traditional route, you will have to at least capture up to 22 or 13 to assault sector 23. Enemies in this sector are well equipped (M14s and M16s with Spectra Shields) so be ready.
There are mines planted around the small islands, especially around Senor Los Makiwahan's headstone, so taking a explosives expert with a mine detector is a plus.
Santino's men will attempt to aggressively regain control of the sector.
Possible Method (spoiler) Edit
If you really need to get the headstone cause of warnings and quitting natives, capture sector 24 ASAP. Then, send your team to the island towards the northwest. Discard unnecessary equipment on the shore so that as little stuff gets damaged. Make sure you bring an explosive expert with a metal detector. Place your team in the water to the west of the island, definitely equip them with knives/combat knives. Traverse your team west to sector 23. Swim (and hopefully they can kill Metavira Eels along the way) until you reach the first northeast island. Don't bother to pick up the detonators on the island. Send the metal detector guy down the bridge, sniffing for mines. The headstone is on the second island, and if the merc is good, he/she should be able detect the mines. Pick up the headstone, send the merc back up, get your team to swim back east to sector 24 and volia! You have the headstone without needing to capture sectors 13 or 22. If you are lucky, enemies should not appear. On the safe side, bring at least one marksman a with old good weapon to act as a sniper. You can assault Sector 23 another time without worrying about the Heastone.
If you have the headstone in one of the merc's inventory by the end of the day, Jack will congratulate you for retrieving the headstone.
The pool of guards and tappers will drastically increase, or max out and remain at 500.