After doing many many runs (on low mp, as i still don't have a ring)
I have found that;
If you go to keywarden area WITH 5 stacks, he will appear a long way from the waypoint, making you search for ages.
If you go to the area WITHOUT 5 stacks, he will appear very close to the waypoint.
Now of course this isn't 100%. But 95% of the time this is how it works for me.
So if keyfarming, go after warden first, find him practically straight away, THEN go stack hunting.
Anyone else found this?