But it's happened to me that it just didn't turn on. Prime time, Cyro was lagging, I closed one and the next one didn't turn on. I had Cyro complete, just to finish the dolmens.
Several times.
And once I saw a marker flash on the map, but those first mobs just didn't show up. So the dolmen didn't turn on.
Yeah, there are *ten* dolmens, and if you have not formally "Discovered" all ten WITH THE CHARACTER YOU ARE ON (i.e. physically been to their location), then there is an X in 10 chance that you will not see a "flashing" dolmen on the map at all, where X is the number of dolmen locations you have not visited with this character.
You may, of course, have Map Pins addon (or a similar one) which shows their locations, grayed out. However, Map Pins cannot tell whether an "undiscovered" dolmen on your map is the active one.