If you killed an I.D.P.D. unit on a level, the next I.D.P.D. spawn will be replaced by an I.D.P.D. Van. Each loop adds an additional Van that will teleport in after the first. The moment any I.D.P.D. portals or Vans appear is randomized and anywhere between 20%-80% of all enemies killed on that stage. For example, on loop 2, the first spawn will be a pair of portals which open after you've killed 27% of enemies, then a Van will teleport in on 45% and a second 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. Deals contact damage only while moving. It can be damaged by enemies that aren't a part of the I.D.P.D..

After a short distance the Van will stop and unload I.D.P.D. troops. It stays stationary after unloading all of them. 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.

Vans don't have to be destroyed to finish a level. The level ending portal will open even if you leave a Van undamaged.

Getting in the van and pressing "E" while it's unloading I.D.P.D. units will teleport you to the I.D.P.D. HQ secret area.