Give the man a break
He's obviously just a pax who doesn't know the technicalities between flaps/spoiler/inspection panels. His description is accurate enough to be able to understand that what was seen was an inspection/access panel. What exactly it's access to is irrelevant in my opinion. As for whether it should have opened, the answer to that is certainly not, but it's not a problem these things do occasionaly open if they haven't been closed properly after regular maintenance. It just requires closing/replacing when you reach destination/maintenance base dependent on whether it's been damaged or not.
Hope that answer helps you out.