Well, I rather doubt someone who would be trusted to work with classified information would have that sort of thing on their phone (though I thought that about them not simply losing laptops and flashdrives full of classified data too) but a lot can be gleaned just from which people are at which places at which times.
e.g. noting that a lot of the people who travel to $CLASSIFIED_LOCATION also seem to stop by $INTERESTING_OTHER_PLACE regularly is valuable all by itself, along with the identities of other people who also go there. All it needs is for people to carry a trackable mobile device in their car and a compliant advertiser.
...
I think years of reading about security and the data trails we leave without realising it has warped my brain; now I'm wondering how many red flags I've set off
