When you come to a tank, get in front of it and look at the small box. On the box are numbers: 030569. The birthday of Marcus Lehto, (the art director for Halo), is March 5th, 1969.
On the bottom of Master Chief's foot is the initals MRL. These are the intials of Marcus R. Lehto, the art director for Halo. The best way to find this is to kill someone in a muliplayer match and take a sniper rifle and zoom in on the body's foot.
If you take a sniper rifle and zoom in on the butt of a pistol, you'll find a weird marking. This is actually a Chinese/Japanese symbol for power.
If you take a sniper rifle and zoom in on the top of a pistol, where the small gray part is, you can find a Chinese symbol for 7. One-One-Seven is Master Cheif's number.
Here's a very hidden Marathon logo. On Keyes, when you find Captain Keyes as a blob, you can find his pipe hanging off the edge of the platform the blob is on. If you take a sniper rifle and zoom in on the pipe, you can find a faded Maracthon logo.