Unrecovered read error on scsi drive

[Don Bowman]

I have a system which popped out this error after many [500?] hours
of disk stress [swapping, dd]:

(da0:ahd0:0:0:0): READ(10). CDB: 28 0 0 c7 92 a0 0 0 28 0
(da0:ahd0:0:0:0): MEDIUM ERROR info:c792bb asc:11,0
(da0:ahd0:0:0:0): Unrecovered read error field replaceable unit: e4
sks:80,86

However, it claims to have 0 defects:

# camcontrol defects 0:0:0 -f phys -G
Got 0 defects.

How can this be? Are there other causes for this unrecovered read error?