Pentagram

The Pentagram is a symbol of hell, which appears throughout Doom games. And a teleporter for monsters in Doom 3.

The Ultimate Doom and Doom II
Pentagram appears throughout Doom I, mostly in hellish levels, there is also a Icon of Sin symbol in the middle. In Doom II Pentagrams can be found throughout the game, signifying that the Earth has been invaded by the forces of hell. Also Icon of Sin is carved onto a large Pentagram.

Doom 3 and Doom 3: Resurrection of Evil
In Doom 3, Pentagram represents a sign of teleportation, as Pentagram spawns on the floor monsters teleport in. Pentagram can also be found written on the floor with candles next to it, referencing Demonic Rituals.

Hidden signs

 * Doom marine's face from The Ultimate Doom.
 * UAC Corporation logo.