Maybe this was all just coincidence. Sometime before beating the main quest, I lost Shadowmere. I don't know if he died, got very, very lost, stuck, something. Soon as I beat the main quest, he was back. I stepped outside city gates, and he was down by the stables waiting for me. Certainly struck a chord. Has similar happened to anyone else? Did I just have a really cool bug? In Skyrim-time, I'd say he would have had to of been stuck or wandering on his own for weeks, if not longer.