On that one, I bet it was getting some sort of interference from the road work sign.
Those often have either RADAR units built into them (so they can be used as "Slow Down!" signs) or what's known as a "Safety Warning System" (SWS) that is meant to work with more recent RADAR detectors to warn you of construction zones or stopped emergency vehicles.
The system was never fully implemented, so it's not universal. As a result, it's rare to run across it. However, when I used to ride with a RADAR detector, I would get occasional chirps from it when approaching construction zones in some places.
It's possible that system is built into that information display that was on the right side of the construction area and it was sending on a close enough frequency to your own vehicle's RADAR to confuse it.