https://www.bbc.com/news/world-europe-50325887

"The incident was described as a GRIP-3 situation, Dutch officials said, meaning an incident or serious event with major consequences to a local population. Regulation documents published by the US Federal Aviation Administration (FAA) explain that pilots can use a special transponder beacon code, typing 7500, to raise an alert for unlawful interference. It remains unclear if this is what happened during the false alarm on Wednesday".