So how do you know the rest of us are not AV professionals
It's really limited to only a few causes
1) Physical damage - e.g. knuckle or something that can give a point pressure enough to break the screen underlying component to break but not leave a mark
2) A screen failure itself - could happen but rare
3) A component underneath the screen that has happened to fail at the same time in two places - possible but you would see reports of it on the Interweb
How old is the TV?
Unfortunately if it looks like fish and smells like fish it probably is fish.
It looks like physical damage and therefore will probably been seen as much. Even claiming under the Consumer Rights Act would be challenging as they could say you damaged it and therefore there liability is waved.
It is most likely an insurance job....