Gear trouble perhaps. Or they are just checking something out after some maintenance procedure.
I live about 15 miles from Paine Field (KPAE) and from time to time, a new Boeing jet flys over low and slow with gear down. I've checked FR24 and its usually a Boeing flight for a new plane just off the line. I imagine they took off, got some sort of warning message and either left the gear down or dropped them quickly to ensure they would be down before a hydraulic problem (leak?) prevented that. And head back for the shop.