On several occasions I've noted that the last enemy (I can't recall if this has happened to multiple enemies) in a wave disappears or is presumably killed by the game if it takes "long enough" for the players to find. This is common on larger maps such as Mountain Pass and Farm. I've also played extensively solo so there is no change that another player finished the specimen.
KF1 was criticized for the occasional need to search for the remaining enemy(ies), as the match won't progress until everything is dead:
[…] Another annoying situation can come up when you're trying to
find and kill the final enemy in a wave. Though they generally succeed
at navigating the map, creatures can occasionally become caught on
something, and nothing is more frustrating than spending several
minutes trying to find the last creature only to discover that it's
invisible and stuck on a log 150 meters away from the trader.
I've played since early 2014. Perhaps the developer Tripwire released an update addressing this issue in the release version?
Best Answer
I don't believe Tripwire ever officially noted any mechanics regarding this in any patch notes. This mechanic was probably used just so people wouldn't get stuck on waves because there are many times that crawlers and clots get stuck on boxes. However for smaller maps, it is hard to get far enough away from certain spawn points in order to despawn Zeds that are stuck.
From my experience (well over 300 hours), once you get around 5 Zeds remaining in a wave, the game will check to see the distance of each zed from the player. If the Zeds are too far from the player, they will begin to die off one-by-one. Many times, especially on Farm, the Zeds will die off while I'm trying to run to the trader that is across the map. Also, there is no limit on which type of Zed can die off this way. I've had scrakes and fleshpounds die from being too far away as well as everything else.
The only source of information that I could find regarding this topic is found here.
The link is a steam forum so I wouldn't hold this to be exact but there are a couple responses about the Zeds dying off, and the only difference in the posts is how many remaining Zeds are there when the game starts checking distances. One person says 7 Zeds and the other says 3 Zeds. Like I said before, I usually notice them start dying off around 5 remaining, but the relative range is the same.