Hinged metal plate attached to one side of the ladder to prevent access to the ladder rungs.
Seen most often on roof access hatch ladders at warehouses.
But that only applies to attached ladders.
I don’t know what type of ladder was present on this site.
If you look at most McDonald’s buildings they’ll have a ladder with a locked metal plate over it leading to the roof.
https://www.fallprotectionusa.com/product/ladder-guard-ladder-door-security-system-bluewater/
Here’s one kind.
It has been said these buildings were close and that he moved from building to building. It is possible we are looking at the wrong ladder that he climbed up.