Mysterious swap partition I can only get rid of on reboot

So, one thing which came out of my dying drives saga is this. I have a
swap partition on each drive. on /dev/ada0p2 and /dev/ada1p2. When
a drive dies, like ada0 did earlier, then swapinfo looks like this:

Device          1K-blocks     Used    Avail Capacity
/dev/#C:0x74     16777216        0 16777216     0%
/dev/ada1p2      16777216        0 16777216     0%
Total            33554432        0 33554432     0%

OK, thats a bit odd. But I stick a new drive in, partition it, and
run 'swapon -a' and it ends up looking like this:

Device          1K-blocks     Used    Avail Capacity
/dev/#C:0x74     16777216        0 16777216     0%
/dev/ada0p2      16777216        0 16777216     0%
/dev/ada1p2      16777216        0 16777216     0%
Total            50331648        0 50331648     0%

...and the thing is that I cant remove /dev/#C:0x74 using swapoff that I have
managed to find. I have to reboot to make it go away.

-pete.