Once both shrines are destroyed, you'll get a call on the radio from Dutch, about a troubled hostage. This launches a Guns For Hire mission.
Head for your new marker. Use your binoculars to tag hostiles, even through the greenery.
There are two here- one menacing the hostage, and one on the far side of the boathouse, keeping watch. In this case, gunning them down is simpler than a stealth approach, and the handgun is accurate enough for the job.
Liberate the hostage to complete the objective.
Press 'R' to hire him, and he'll follow you around, and aid in firefights.
Return to the Research Station, which is now overrun by hostiles. This is a chance to test the sidekick system in action.
One completely viable strategy to send your partner up ahead, then flank and cut down enemies as they converge on him. No matter how you wipe them out, their destruction completes the Guns For Hire mission.
There's another captive inside the station itself. Free him for some RP, and for the location of the local cult outpost.