The UBL G29 P1 is smart about not running the probe off the bed. You will end up with a few rows (and columns) in the mesh map that are not automatically populated with the Z-Probe.
G29 P3 T (maybe twice depending on your mesh resolution) should fill in the unprobed locations.
G29 P3 T (maybe twice depending on your mesh resolution) should fill in the unprobed locations.