Time to close the panel on this one, I feel.
No it should not have been open - most likely an engineer's error, but it could have been a failure of the fasteners.
It is very difficult to check all the panels on an aeroplane - counting wings is enough for some
It would almost certainly not cause a problem in flight, but panels coming open/falling on people on the ground - not good.
If you see this again, ask the cabin crew. They should be able to judge whether or not a pilot needs to come and have a look
I think we can close this one with Mairi a little wiser. If she wants to learn a little more, this excellent web site will help.
http://www.b737.org.uk/flightcontrols.htm