Advance guard

As observed by the Thirteenth Doctor, Cyberdrones served as an advance guard for the Cybermen during the Cyber-Wars. (TV: Ascension of the Cybermen)

Just prior to the Battle of Canary Wharf, what the Tenth Doctor observed to be an advance guard of Cybermen from Pete's World in 2010, led by Cyber-Leader One, infiltrated the Torchwood Tower of N-Space in 2007, where they provided an opening for an army of five million Cybermen to cross over from their parallel universe. (TV: Army of Ghosts)