The sign was originally part of a contest. The first person to find the sign and do as the message said would receive a prize. From the Apogee FAQ:
The sign was to be the goal in a contest Apogee was going to have, but almost immediately after the game's release, a large amount of cheat and mapping programs were released. With these programs running around, we felt that it would have been unfair to have the contest and award a prize. The sign was still left in the game, but in hindsight, probably should have been taken out. To this day, Apogee gets letters and phone calls and asking what Aardwolf is, frequently with the question, "Has anyone seen this yet?"
Subsequent releases of the game including the Steam release have had the sprite file changed to an unrelated image.
The word "Aardwolf" was presumably chosen for obscurity: an aardwolf is a small African mammal.