The Walls have Ears... and Eyes!
This enhanced level of difficulty should be worked into some missions: The Clockworks (or Crey, or some other technology-savy villain group) has taken over office building X. They've also tapped into the building's security system and are watching the heroes' every move on the many security cameras set about the building. They know where you are... They know where to send reinforcements. Basically, the building's security cameras become targetable and act as ambush-spawning devices until they're disabled (working much like the "protect the artifact" missions). Once heroes come within visual range of a camera, unless they destroy it quickly, villain ambushes will spawn at regular intervals. When they're disabled, no more ambushes.
