This is a headcanon that shamelessly builds on canon and is, thus, not entirely justified by it.
The Fëanorian Noldor, after the Dagor nuin Giliath, developed the custom of burning their dead and scattering the ashes in clearings located either between the mountains or in the hills. Naturally those clearings assumed thus a great significance for the local exiles and so, as a way to signal such a place to each other, the Noldor started planting three cypresses huddled together in their center.
This went on for the whole of the first Age and some surviving “loyalist” kept the tradition well into the third, before passing or sailing.In Fëanor’s case there never was a grave because the wind swept everything away, and it almost became part of his mythos: for his followers somehow believed that he could not have a final resting place as long as there was a fight to fight. Still some of his sons do have a “grave”, albeit a secret ones. After Celegorm, Curufin and Caranthir died their bodies were burnt and their surviving brothers scattered their ashes in a clearing deep inside the Blue mountains, near mount Reirir. The same happened for the bodies of Amrod and Amras, they were recovered, burnt and their ashes laid to rest in the same place.
The ones who know about this spot are few and become fewer and fewer as time passes, still the remaining “loyalists” that still search for Maglor have traveled to the spot several times, since it survived the ages and the flooding of Beleriand, and always found the trees in its center somehow “cared for”.
So much that there is the legend that as long as the trees are alive so will be Maglor.
