Hawk kept scouting as they returned to base, and Karl realized that while they got closer to the base, the monsters became more sparse and weaker. Groups of five and ten Goblins at the Common Grade, a single lizardman, and threats at that level were all that the short range scouts of the base would have found.
But the further he flew away from the base, the stronger and more numerous the monsters became.
So, the problem wasn't that they had found unexpected monsters, it was that the limited scouting that the base had done hadn't given them a proper expectation of the monsters which were in the region.
The Captain had determined this as well, and as they approached the base, everyone could see his resolve to have a "discussion" with the base commander and the head of intelligence.
They had flown multiple helicopters over the region, they should have some sort of idea what was there, but it seemed that nobody was even looking out the windows.