It was unlikely that his enemies would see what was being done, so unlikely that they did not think to prepare. So unlikely that, appropriately enough, it was mentioned in chapter 78:

The Dragons had started the combat with a feint to provide a distraction for Mr. Goyle's approach through the forest; Neville hadn't realized there were two brooms attacking until almost too late. But the Chaos Legion had gotten the other pilot. That was why broomsticks usually didn't attack before armies met, it meant a whole army would concentrate fire on the broomstick.

The Dragons sacrificed a broom to see what was up. This is a significant sacrifice as they have only two and their enemies have a total of four between them.

It would not be surprising for the next battle, if there is one, to include some protection against aerial espionage. Better spy-planes would be another reasonable result of this battle. The first thing you do is fly higher.