I'm not sure, to be honest. The stable marker/city stable scripts (as you probably expect) use AddScriptPackage with a StayPut package on all the stabled refs fairly often to keep them "tethered". In beta testing, all manner of things were tried to "break" the process, like waiting and restarting the game and pcb'ing etc... over various time intervals ranging from a week to three, as well as just plain playing the game while various horses were stabled in various ways in various places. However, just this week, I lost a horse from a stable marker after playing through some of Tears of the Fiend, which required my character to take far more naps than she otherwise would. This varies from the several game weeks I spent beta testing another mod. During that time, my character was mostly sequestered in that mod's location, but Shadowmere was where I left her when I got home. :shrug: In any case, I has some thinking to do. Input welcome!
Interesting. You know, in my case, the gate to the corral was closed and so was the gate to the courtyard, so I really suspect the game teleported PM's horse to Weynon and he didn't walk back.