After lvl 12, i found it in Bleak Falls Barrow. I never did the main quest lol.
1st was with the companions, then in broken oar, i use the dawnbreaker constanly, its the skyrim equivalant to exalibur
I found the Beacon in the chest near Cicero during the DB questline when you can kill him. Pretty weird how it randomly spawns in the worst places.
In a Giant Camp
I was trying to get a mammoth tusk for Ysolda (I wanted to marry her) and found the beacon on a chest.
Shrine of Mehrunes Dagon.
heehee!