The Containment Unit Escapees are ghosts who have escaped from the Containment Unit or have been released by others.
The Real Ghostbusters
Ever since the Ghostbusters were founded, entities who were captured have long since escaped, were released by accident, or released by intent. The Containment Unit was once infected by the essence by the Grocery Specter, it animated and spit out several ghosts. The Ghostbusters were scared by a Containment Unit alert and ran down into the basement. Peter Venkman became concerned and wanted to shoot the unit. Egon Spengler stopped him and quickly went to work on a Particle Thrower modification to extract the harmful effects without breaching the Containment Unit. Winston Zeddemore and Ray Stantz trapped all of the escapees and used nearly all of the empty Traps available.
After the Grungy family bought the Firehouse and moved in, Foul Grungie mistook the ghosts in the Containment Unit for squatters and tried to kick them out. Several dozen ghosts escaped the unit before Foul jammed the trap door closed. The ghosts hung around the front entrance of the Firehouse and attacked the Ghostbusters. Outnumbered, they ran into an alley and lost the ghosts. Once they adopted disguises, Ray and Egon strode past the ghosts and entered the Firehouse.
Later on in 1997, ghosts were released by Roland Jackson while under Siren's hypnosis. The rest of the Ghostbusters including Janine had to deal with escapees, Donkey ghost (which can breathe fire) and Gas ghost. Roland took Banshee to her sister Siren after releasing her during the same encounter.
While trying to put away Fenris in a Trap, which had been compromised with ice, it opened while being injected into the Containment Unit. A number of ghosts were peeking out trying to escape. Slimer flew into the group of ghosts, pushing them back into the Containment Unit as he went in with them. He then closed the inner hatch from the inside.