Crystal Desert boss spawn points in explorable areas

When I explore new areas usually I don't carry a Signet of Capture with me because I don't want a skill bar slot occupied with a skill that's unused 98% of the time. But recently a few times I've come to regret not having an SoC at hand:

In the Crytal Desert I've run into quite a few interesting lvl 24 bosses and I would have loved to check if they got any skills worth capturing. Unfortunatly, when I grab my SoC and return to the same place I never found the same boss in the same place again. For example I've once stumbled across a monk hydra boss just outside the western exit of Augury Rock. (Can't recall the name.) I must have zoned in and out of Augury Rock about 15 times checking the spawn point of that boss, but ... nothing ... nada ... niente ... ne rien ... nichts.

Are the spawn points for bosses in the explorable areas completely random? What's the chance for a boss to spawn? And do they spawn in the same general area or even in the exact same location?

Please note that I'm not talking about the well-known elite skill bosses in the missions. I'm talking explorable areas here.

No idea on the random chance, but yes the spawn points are indeed random. As for the boss hydras, those guys like to wander far and wide within the zone they spawn in and they seem to have their own version of the aggro circle which is way bigger than yours as they always seem to head straight for me at a run even when they're nowhere near my aggro circle

For elites skills in the dessert I use this map .
This post has link to more areas & one big map with all elite skill locations (I think).
For regular skills there isnt a map.