Spent grains are an odd entity to me. I have a compost pile in the back yard that I put all my grain in. It's open air and I usually just dump it on top. There is a nature preserve behind my yard, woods, swampy, full of wild life, coyotes, skunk, raccoons and a ton of squirrels are all over. Even during the harsh winter we had this year, I never noticed anything eating the grains that were sitting on top, They look exactly the same as the day I dumped them. No footprints to be found anywhere. IF nothing else, I figured the birds and mice would at least get to them.
I wonder if we are pulling enough of the sugars out of them, that the critters look at them more like wood pulp than really a food source. I know one time I accidentally spilled some unused grain walking it from my car. That was a torrent of activity for about a day, until it was all gone.