That's a good question that I've been pondering myself over the past couple of weeks.
He shows up with a journalist and a bodyguard/assistant and then... vanishes.
Some say he was killed outright.
Others said he was languishing in a Libyan prison somewhere.
After the revolution a delegation from Lebanon briefly spoke with one of Ghaddafi's intel chief's who'd been detained in Mauritania. He was evasive and never really gave them a straight answer.
Here's a couple of articles on the subject:
And finally, a book about the subject.
I'm quite curious as to what sparked you interest in this subject.