Help!  I've had this problem before maybe a couple of years ago.  I thought it was voles back then.  Raccoons?  They were fine maybe a week ago when I last took a good look at them.  I did have a problem recently with cabbage worms but I hit them with pyrethrin and they were gone.  They did very little damage.  Now this??

That definitely looks like it, but it's not possible.  It's in my 'upper' garden that is raised about 4 1/2 feet off the ground and right up against the house.  They would simply never go there and there is a big patch of vegetable marrow between the edge of that garden and the cabbages.

Game cameras are pretty cheap these days and worth their weight in amusement.

listen for bigfoot farts in the night.   it gives them awful gas,,,,,   might be deer.   moose get into ours, the  tracks are un mistakable thou

I don't have the gardening/farming experience you have, but I'm completely stumped as well.  

I did have problems with voles one year eating my carrots, and I had the same type of damage to my cabbages about 3 or 4 years ago.  But a vole is a little on the small side to do this kind of damage to it.  

Along your lines of not finding any evidence of mammals or insects, we have had a lot of rain lately.  I just did a quick google search for "cabbage bursting due to heavy rain" and got these threads:

http://www.vegetable-gardens.co.uk/forum/seasonal-vegetables/1438-bursting-cabbages.html

http://www.farmersweekly.co.za/article.aspx?id=422&h=What-causes-cabbages-to-crack

OK, here's a picture when googling "cracked cabbage".  I think we have solved the mystery.



From this link

I feel good because it's not contaminated with some animal's mouth, like rabid raccoon saliva.  We had a crazy amount of rain for the week or so before I noticed this.  From the brief reading, it doesn't have to be tons of rain, but it is usually when the heads are full-grown that they are most susceptible to this.