If all you see is paper husks, it sounds like they died a while ago. Like any 'dead body', the longer it is dead, the harder it is to determine the cause of death.
Do you know if they were alive last October? Did they look healthy and normal?
If it were borer, you may not have seen them in the late spring, early summer. There may have been holes in the iris, and devastating damage that could not withstand the winter.
if you are sure they were healthy in the Fall, there are other causes of winter death. An animal may have eaten the roots, or it could be winter fungal rot. They may have heaved in the freeze/thaw cycles.
I always have a little winter mortality, but only a small percentage die because of the harsh conditions. Maybe 1%.
The best thing you can do now, is make sure you apply some Merit in the early spring, and observe carefully. By Fall, before the rhizomes bunk in for the winter, make sure they are firm and free of holes, or mush. Then you know its not borer. Examine a few times during the winter to see if you can detect something abnormal. A healthy 'hybrinating' rhizome is hard and anchored securely in the soil.
Good luck,
Laetitia

Empty shells, like Laetitia mentioned, sounds like something triggered a rot infection a while ago. It could have been borers last summer that chewed holes in the rzs, leaving the leftovers susceptible to rot. Or it could have been any one of 100 other stressors that leave rzs vulnerable to rot. Whatever happened, happened long enough ago for the soft tissue inside the rzs to have rotted completely away.

If I were you, I'd wait until after iris bloom is finished (assuming there are still a few irises in that bed that may bloom this year), then dig any remaining plants out and give the bed a makeover - weed it, amend it, raise it if it tends to hold moisture, etc.

Treating immediately with Merit is a good idea since you do live in borer territory.

Laurie