Had a panel fly off a 747 wing in a similar position during the climb - long story, ultimately proved to be a fibreglass fillet panel but one didn't know, nor what had punched it off, gear or flap retraction etc. so I dumped fuel and landed back at the departure airport in fairly short order.
At the engineering enquiry Boeing asked why I had returned, as the 747 could fly with a panel 6' x 9' ( forget the exact dimensions ) missing in that area (Oh, really, news to me ).
The company engineer said " What's the Captain supposed to do, go out and measure it ? "
Too right, I was back on the ground, fix it.