New from 404 Media: the FBI has been unable to get into the iPhone of raided Washington Post journalist because the phone had Lockdown Mode enabled. Apple markets Lockdown Mode mostly to stop spyware like NSO. Here, a real world example of it stopping access too https://www.404media.co/fbi-couldnt-get-into-wapo-reporters-iphone-because-it-had-lockdown-mode-enabled/
FBI Couldn’t Get into WaPo Reporter’s iPhone Because It Had Lockdown Mode Enabled

Lockdown Mode is a sometimes overlooked feature of Apple devices that broadly make them harder to hack. A court record indicates the feature might be effective at stopping third parties unlocking someone's device. At least for now.

404 Media

@josephcox “The iPhone was found powered on and charging, and its display noted that the phone was in ‘Lockdown’ mode.”

That's interesting. Lockdown mode doesn't normally indicate on the lock screen. They might mean that some action was initiated that lockdown mode blocked—when that happens, you get a notification that says "lockdown mode blocked xxxyyy." That notification might be the "indication" they mean.