Phantom deleting of friends fom my list

Question

ok. I have a friend on my list who has disappeared twice. I havent deleted him and i dont think he deleted me.... there are 700 people on my list and he is the only one gone. can this be due to a facebook problem/error?

The only other explanation is that we have a hacker who is getting in to one of our accounts and deleteing the other one. Is there any way of finding out who deleted who and when? This would be important because it is likely to have happened during a time frame when we were both out and in the same place.