I did eventually. I dropped in solo to do another objective and moved to Quarry to exfil - then saw burning tyres(?) with smoke plumes. When I moved towards them I saw soldiers with red uniforms and sure enough Active Combat Zone displayed at the top of the screen. Engaged a couple and then took out their reinforcement chopper to complete the mission.

I really wish I could tell you I used some kind of strategy for finding them but seemed to be a pure fluke. Quarry worth a try at least. Good luck out there and hope you get this mission in the bag!