This is straight up classic police legwork...
Which may be built on the back of a massive privacy violation. Should location data be made available to the government when they don't have a warrant? Should they be allowed to "go fishing" with our data?
> they weren’t suspected of anything but the agents asked for any photos they had of the events.
There are plenty of other methods available to the FBI to achieve this goal.
Now they show up to your house telling you they “pinged” your phone near a suspected crime “but your not a suspect, we just want to see the pictures on your phone”. Refuse to show them, and they already know you posted one or more (maybe subsequently deleted it) and maybe it’s obstruction of justice. Deny you have any, and it’s lying to law enforcement.