Chris noticed that on one of our french door hinges the main screw was not screwed down and the rubber cap was well proud of the hinge. I screwed the bolt down so that the rubber cap would seat properly. See images below.
However Chris was concerned that if it was that easy to screw in the bolt which forms the hinge, it would
be easy to unscrew these on all three hinges and remove the door!
On investigation it turns out that there are "security grub screws" in each hinge, located so that they cannot be accessed from outside. These are to prevent the main bolts being turned once adjusted.
The worrying thing was that every one of our security existing screws was loose, one was missing, but luckily I had found it when we moved in and put it away safely in case it was useful. All required several turns with an Allen Key to tighten them so that they secured the main bolts.
I suggest that everyone with this model of French Doors checks that the security grub screws are tight.....
(This is most definitely not my area of expertise, and if anyone knows more about these doors and their construction, please let me know).