Radar isn’t going to be very useful for small things flying at, or even below, tree level. Everything down there is ground clutter. It’s odd that people seem to just assume that there are sensor systems that can just capture anything and everything. There aren’t. At sea they might have a chance to pick it up but even then they might not. Ashore... Not likely at all.
Washington DC is covered by a redundant array fixed radar system. All ground clutter is mapped and eliminated from scan. If this Gyro wasn’t seen it was because of improper reading of the scan.