Signs are a form of Easter egg in the Grand Theft Auto series. They take the form of messages that were deliberately placed by game developers for the players to find, and they are usually ironic or humorous. They have always been hidden in secret and hard-to-reach places. There is always at least one sign in each game that sometimes leads to other Easter eggs.
On the top of the Gant Bridge (one should use a Jetpack or a plane to reach it), there is a sign that reads "There are no Easter Eggs up here. Go away." The Gant Bridge Visitor Center nearby also has other humorous signs regarding the bridge, such as a disc space of 1.27MB.
In the cemetery in Hashbury, San Fierro, all of the gravestones read "RIP OPPOSITION 1997-2004." This is a reference to Rockstar's rivalry with other video game companies, and how they have not been able to release a video game better than GTA since 1997 when GTA 1 was released.
There is a sign next to the bridge facts sign saying "ACTUAL PIECE OF CABLE."
There is some graffiti writing in the Los Santos Cemetery that can be found on a wall behind the mausoleum. It got the nickname Ghost Graffiti, as it appears at the in-game time of exactly 20:00 and lasts until 06:00 in the morning, which is when it disappears.
Around San Fierro, there are signs with "UNBELIEVABLY STEEP" written on them, remarking how big the differences in height are in the terrain that the city was built on.
Zombie Easter egg.
Gant Bridge Easter egg.
"BRIDGE FACTS" sign.
"ACTUAL PIECE OF CABLE" sign after the bridge facts sign.