I found a patch of ground that took me out of stealth the 4 times I tested it out. This link is to a video showing the stealth loss occurring twice. I forget the keep name, but those in the know I hope recognize the site.
vid says Kingscrest, u sure it wasnt the damage from your stone treb taking u out of sneak as travel time would probly make it hit just as you were entering sneak? dont know if u were usin treb every time u tested that patch of ground
Dovie'andi se tovya sagain.
Niidro tiid wah fusvok dirkah.
No resistance from keep, no detectable damage from treb (or damage from anything else I was aware of). I was cycling between treb each time I crossed the spot as I was on the look out for trouble.
I had dropped out of stealth two additional times prior to becoming aware of the spot; I repeated it four times to make sure I was not taking any damage or NPC were no messing with my stealth.