I finally saw the culprit - I grabbed a flashlight and searched around through the false paths I could see into, and finally saw a gleaming, golden eye - a pretty brown toad staring sullenly at me :) Neat!
I'll have to install some "toad home" pots around that area next year, I don't particuarly WANT them digging my plants up, but they're not hurting anything. The huge furry beasts that dig up my egyptian onion bed and then lick my face are more upsetting.