It was actually one of the longer bolts for the 10mm ECS spacer. Now I'm thinking that's the real culprit as I would never leave a wheel untorqued...buuut I would also go several months without checking the torque on wheels with aluminum spacers between them and the hub.
My guess is that the hundreds of heat cycles from driving caused enough expansion/contraction of the spacers to slowly loosen the wheel bolts.
And then one broke...
And I didn't notice...