The following sections show cropped map images for Serious Sam's Bogus Detour denoting the locations of the three "official" Easter Eggs. Two are tracked via Steam achievements:

Duke Nukem
Meet up with the Duke.

and

Mr. Buttersworth
Find Mr. Buttersworth.

the third one is random and relates to Portal's Companion Cube.

Note: The Easter Egg location data was derived directly from the editor that comes with SSBD. Added large arrows to the maps to make it easier to see where the EEs are even on a scaled down view of the maps.

Note²: The Easter Egg maps are cropped, scaled down by another 50% versions of my full Maps for Serious Sam's Bogus Detour Steam guide. Be sure to check it out for more details.

AEon

In the south-west of the map, you will have to navigate a slightly hidden path between buildings to make it to Duke Nuk'em and his bike. Getting there unlocks the "Duke Nukem" achievement.

From the south beach, make your way north until you reach four cages on the eastern side of the map. Get rid of the turtle in the northern-most cage, enter the cage and find a path between the trees just north, then head east to picnic area. This unlocks the "Mr. Buttersworth" achievement.

This Easter Egg features a very brief cameo by Portal's Chell, who opens a portal and places the Companion Cube in the map using her portal gun. Note that this is not tracked by an achievement. The cameo will occur at random in one of four locations in the map. In some cases it is enough to walk past the locations, but since the cube when shot will yield a star you might also actually want to get to it. Since all the locations are related to secrets, the descriptions are a bit more detailed.

1. This one is located in the very north of the map in a locked-off passage. Head to the NE of the map, shoot the wall there, take the teleporter to a small room, shoot the south wall, shoot the blue floor button, exit south, shoot through the gap in the wall (northward), to open the locked door to the passage. At the northernmost part of the passage the portal may appear.

2. To get to the secret area with the four biomes, first you will need to get the eye of a blue-suited scientist from the north sector of the map. Then unlock the gate west of the central hub, take the teleporter to the biomes area. Press the switch on the SW biome, wait, and then enter... the portal may then appear here.

3. In the north-east part of the biomes (see 2.), blow up the wall into a secret corridor, take the teleporter there to a previously locked off room just east of the central hub with many confusion traps. Shoot the blue floor switch, to unlock the small room in the SW corner... a portal may appear here, or more likely by passing the room the Companion Cube may already be there.

4. The last location is accessed via a hidden teleporter that is south of the exit trigger area, take the teleporter to a previously locked off set of rooms... a portal may appear there.