Not a big issue.
Has happened to me three times since 0.15, have cleared out an area of biters and then many hours later when going out to a cleared area I'll find a biter just standing there. However when I approach the bitter it just Disappears.
No mods or commands used on this save.
The biter is clearly visible and can be selected to see it's hit points.
It does not react to buildings placed near it and it only disappears when the player character approaches it.
It does not perform a death animation or leave a corpse behind it just disappears.
Using default railworld settings.
I clear areas by cutting out a square of space around a pack of biter nests say 1500*1500 tiles then go around the square from the outside in killing everything.
I believe some biters are getting missed due to random movement + my site radius and then are to far away from any living biter nest to find a new home so they just sit there until they "time out".
