Before everybody jumps to blaming maintenance (unless they've got the little baggie with the missing screws in their hands - naughty), it's well to remember that over the years,the 747 has shed quite a few wing leading edge and trailing edge fibreglass panels. The panels being fibreglass can be subject to cracking from moisture ingestion ( freeze / thaw ) in minute stress cracks. They're not generally visible to the naked eye, especially on top of the wing and need a "tap test" with metallic coin of the realm to see if they're cracked. They tend to lose wing inboard trailing edge panels more than the L/E, but unfortunately I've had to fix both over the years. So while its certainly possible somebody forgot to screw it back on, there are a couple of other reasons for said bit to go walkabout.