Harvey Smith, co-creative director of Arkane Studios, @Harvey1966 on twitter ( https://twitter.com/Harvey1966 ), explained the following:

Small Q: What do enemies do after "Should be someone on guard here"? Become alert or just take missing guard's patrol route?

https://twitter.com/Harvey1966
change routes

Any chance on getting them into active "Search for Missing Guard" Status (alert status 2/3) via patch for harder difficulties?

@Harvey1966
probably not; we want them to do that only if they see/hear evidence


For all the people like me that wondered what exactly happens when a guard notices that his buddy is not on station anymore.

Time to play with the controller upsidedown blindfolded for that extra rush.