On the "Last Harvest" map, make your way through the supply cache cave and go to the poppy plantation. When you get there you will encounter between three and five enemies. Quickly pick them off and make sure the field is secure. Do not sabotage the ventilation yet. Go back to where you first entered the field and turn around. You should see a faint rainbow towards the back of the field. Follow the rainbow all the way to where it hits the ground. There will be a small cauldron of gold. It gives you 50 CE, and takes ten seconds to "defuse". It is counted as hidden objective and is worth no extra CE at the end of the mission. Note: This can only be done once, even if you reset the campaign.