Why would lockdown mode prevent this? I have lockdown mode on but that doesn't automatically make my notifications private.
But most likely (pure speculation mind you), this was a case of someone handing over the phone for review and where cooperating.
It might have been that they deleted signal some time ago, or even deleted signal and then handed over the phone.
It's notable that the data wasn't recovered from signals storage (was the data securely erased or that kind of recovery not attempted?).
> Web browsing: Certain complex web technologies are blocked, which might cause some websites to load more slowly or not operate correctly. In addition, web fonts might not be displayed, and images might be replaced with a missing image icon.