This egg is hidden really well. If you go to portal\materials\models\props_facemovie\wet_floor_sign folder of the game, you'll see a lone file called wet_floor_sign.vtf. Open it with a VTF File Viewer, like VTF Explorer and you'll see an entertaining meme. I haven't encountered that sign in the game but if I do, I'll post a picture of it here later and where to find it.