Okay, the game does respawn enemies in areas, as long as you haven't triggered some invisible condition.
 
I walked up to the entrance of a base. Inside were three soldiers. I just peeked in close enough to take one out with a silenced shotgun. A moment later, he respawned right in front of me. I shot him down again. Then I walked inside and shot the other two soldiers. Then I walked past their bodies and turned around. Behold, there was another respawned soldier. So I shot him too. 
 
Here's my video of this particular phenomenon. It's helpful that the game checkpointed right before the enemy group. Three soldiers magically turn into six kills. Very sloppy work by Crytek.
 
  

Okay, the game does respawn enemies in areas, as long as you haven't triggered some invisible condition.
 
I walked up to the entrance of a base. Inside were three soldiers. I just peeked in close enough to take one out with a silenced shotgun. A moment later, he respawned right in front of me. I shot him down again. Then I walked inside and shot the other two soldiers. Then I walked past their bodies and turned around. Behold, there was another respawned soldier. So I shot him too. 
 
Here's my video of this particular phenomenon. It's helpful that the game checkpointed right before the enemy group. Three soldiers magically turn into six kills. Very sloppy work by Crytek.
 
  

Why is this surprising or significant?

It's generally not a good thing to spawn enemies out of nowhere. It was also something that they avoided with the first Crysis.

That seems more like a bug than by design, but yeah, that's pretty rough.  
 
Homefront was the worst about this, though, and by design. Dudes would literally spawn three feet away from you out of thin air, over and over again, because you hadn't crossed that invisible threshold.

" @Delta_Ass: Fair enough, but it isn't unusual for modern shooters to have respawning enemies, so I don't think I can hold it against them. Yes, your video makes it look pretty bad, but to be fair you are tweaking what appears to be a slightly busted stealth mechanic pretty far out of its intended form of use. "

" I have seen guys spawning right in front of me. But the worst is at a mission where I was in the back sniping, and the enemies never stopped spawning. "

meh- I feel like the spawning and re-spawning of enemies in FPS's is pretty standard issue by now.  certain other games had unlimited re-spawns until the player advances.  HOWEVER, I have not played crysis but I did play alot of Far Cry (not Far Cry 2- that's not crytek's work and so doesn't count to this argument).  Far Cry made use of specific enemy counts in specific places- each time you played through a section.  This would then seem a departure for crytek- but not for FPS games.  For the record- the way you could memorize enemy movements in Far Cry and work out optimal strategies was cool- so I see how this would be a downgrade.

There are aliens in this game, so it is possible that it is a part of the story.

" Why is this surprising or significant? "

I think that's Crytek's way of calling you a pussy.