If you mass produce these things you are going to leave traces of your logistics supply chain. Even if you brought them into a staging are one at a time your frequent trips would lead back to where they were made or by whom.
And since China makes like 99% of the parts used in drones, they very well could already have a couple hundred of those lying around and we’d never know.
My nightmare scenario is someone flying hundreds of these in formation on a stadium full of people. They could be launched from a parking lot from a single van or pickup truck. If you sacrifice the ability to hover by giving them wings, you might get very high range and not have to risk yourself at all while launching a devastating attack.