Anyone ekse encountered this. I was in a location where i dismounted shadowmere , traveled a little, got encumbered, used ardvak to fast travel and this causes shadowmere to leave/disapear? He sonetimes reappears outside castle volikar?

I use Arvak sometimes to break the chain when I don't want the horse I own to follow me when I fast travel. It remembers the last horse you rode and makes that horse fast travel with you. I've never had Shadowmere but it sounds like the same thing.

It's normal. I used Arvak to have Shadowmere, Frost, and Generic Horse all at Lakeview.

So where does the old horse go?

The old horse normally goes back to whichever stables you bought it from - my horse normally ends up walking back to Riften stables even if I cast Arvak at Lakeview. I suspect if I want a horse in the Lakeview stables I need to get a steward and have them buy it.

Shadomere I think stays exactly where you left her/him. Eventually the horse will go back to Falkreath near where you first got her, she will do that if you do not use the horse for a long time. i usually leave Shadowmere at one of the Hearthfire house stables, and about 3 times I have had to go collect my strayed horse from near the Falkreath sanctuary, so she is not vanishing very often.