I seriously don’t expect that I will ever be in the situation where govt would demand that.
If that would be my threat model, i would just stop monitoring the location and accept the chance that I lose it.
I survived the last ~20 years that I owned a phone that way, and never lost one
If you need an additional protection layer, you can still try with some obfuscation. A good start would be to not use “My phone’s location” as title
I’d would amuse me to send myself mails of the “helo, im Olga from Moscva, looking sirius men for marry” kind, and have a semi-plausible random location in the EXIF data, but then hide the real location data with steganography in the (obligatory) image itself. (Caution, if you do that last step wrong, e.g. by using the same image every time, you might shoot yourself in the foot very hard, at least if the agancy is worth their money and notices that not only the EXIF data is different… )
Unfortunately, nobody would ever appreciate the cleverness of this, so rather than implementing it, I’ll just leave it here
Admittedly though, an encrypted spam mail might raise some suspicion
DISCLAIMER: This was purely for amusement. Don’t try this at home, kids and whistle blowers.
On your other remark: NFC is way to complicated. Just have a threshold of ~100m position delta before actually sending an update.