Searching before posting, I found this discussion: https://forum.opnsense.org/index.php?topic=5121.msg21235#msg21235
The context here is different. In a fresh OPNsense installation from the amd64 VGA USB version to a system in which OPNsense has been used to totally reformat the drive (in manual mode, but accepting each default), the resulting system boots, but with this complaint:
QuoteAug 8 13:11:25 OPNsense kernel: Trying to mount root from ufs:/dev/ufs/OPNsense_Install [ro,noatime]...
Aug 8 13:11:25 OPNsense kernel: GEOM: da0: the secondary GPT header is not in the last LBA.
Aug 8 13:11:25 OPNsense kernel: GEOM: diskid/DISK-4C531001610425107534: the secondary GPT header is not in the last LBA.
Aug 8 13:11:25 OPNsense kernel: GEOM: diskid/DISK-4C531001610425107534: the secondary GPT header is not in the last LBA.
Aug 8 13:11:25 OPNsense kernel: GEOM: diskid/DISK-4C531001610425107534: the secondary GPT header is not in the last LBA.
Aug 8 13:11:25 OPNsense kernel: GEOM: diskid/DISK-4C531001610425107534: the secondary GPT header is not in the last LBA.
This looks like an inconsistency between the FreeBSD tools used to set up the drive, and where the FreeBSD kernel expects the secondary GPT header to be. Will this be a problem if trouble develops with the primary GPT header? Is there a utility which can adjust the secondary GPT header's location to make the kernel happy on next boot?
I did a second full installation on the system, just to be sure it wasn't a one-time problem. It showed up both times.