When Carmen is in the large cavern with the 'brain bug', Zander passes her a knife, and she puts it in the sand underneath her body. Then she is picked up and tossed forward, without the knife in her hand. Then she miraculously has the knife again, to cut off the brain bugs probe.
I have the DVD version. I looked at the scene regular speed several times, and slow speed a couple of times. She definitely has the knife palmed in her right hand. Her index finger is covering the point as the "bug" pulls her up and tosses her forward. The handle can be seen behind her wrist to one side. I believe she kept it somewhat hidden so the "bugs" would not see she had it.
After I read this I watched the movie again and she DEFINITELY takes the knife with her when she's moved. Yes, I'm positive...I watched it a few thousand time...