The number of I.D.P.D. Vans that spawn on a level is equal to the current loop. After a number of enemies randomized from 20%-80% have been killed on the stage and at least 1 I.D.P.D. unit has been killed, the I.D.P.D.Van is able to spawn. For example, on loop 2, the first spawn can be a pair of portals which open after you've killed 27% of enemies, then a I.D.P.D. Van will teleport in on 45% and a second I.D.P.D. Van on 78%. All of the % values are randomized from 20% to 80% and are different for each area.
It will teleport in through a big I.D.P.D. portal, breaking walls and running over everything in its path including walls and other enemies. It only deals melee damage while moving and can be damaged by enemies that aren't a part of the I.D.P.D.
After a short distance the I.D.P.D. Van will stop and unload I.D.P.D. troops, staying stationary after unloading. When destroyed it creates three I.D.P.D. grenade-like explosions and drops up to 3 ammo/health pickups. It can be destroyed while it's moving which will prevent it from unloading troops.
I.D.P.D. Vans don't have to be destroyed to finish a level. The level ending portal will open even if you leave a I.D.P.D. Van undamaged.