Why can't i see gps metadata from someone's else photo?

I read about this in Mark Goodman's book:"Future Crimes" and i wanted to try if this actually works. It doesn't for me. I tried
my iPhone photos (even turned on location accesses for camera) and i just can't see gps coordinates.
Do you maybe know what is the problem?

Depends on how the image you are looking at is saved. The raw GPS metadata is saved in an extended header. But if you post that image online then often you will not post the extended header information, but only the file and it's file name.

So if I post an image to FB or twitter it'll strip off the header, but if I post the same image to Flickr it won't!