I'm guessing this is an "easter egg" in the Medal of Honor: Allied Assault game...

When escaping from Trondheim (where you destroy the Naxos prototype and sink one of the submarine U-boats), the last section of the mission starts with you in a vent duct. Well, you're not alone up there and a little ways ahead of you (and moving as fast as you move) is the elusive Trondheim Duct Vent Slug.

Medal of Honor: Allied Assault - Escape from Trondheim, chasing the vent slug Medal of Honor: Allied Assault - Escape from Trondheim, chasing the vent slug

Medal of Honor: Allied Assault - Escape from Trondheim, cornering the elusive vent slug Medal of Honor: Allied Assault - Escape from Trondheim, cornering the elusive vent slug

Medal of Honor: Allied Assault - Escape from Trondheim, the invincible vent slug

I wasn't able to find anyone else reporting this easter egg via google. OTOH, querying for +"medal of honor" +"allied assault" +"easter egg" escape vent duct came back with nothing but spammer sites for at least the first 15 pages worth of results.