D'oh. I'm a bit tired, clearly.
I found this in vmcore.7:
Code:
<118>Checking for core dump on /dev/ad4s1b...
<118>savecore: reboot after panic: ffs_clusteralloc: map mismatch
<118>Oct 9 11:16:26 freebsd savecore: reboot after panic: ffs_clusteralloc: map mismatch
<118>savecore: writing core to vmcore.6