To be fair part of the warning should be to keep a close eye on your phone/make sure it's well locked up because it would take someone who knows what they're doing 5 seconds to slip an undetectable GPS locator app on there and disguise it as something innocent... and then you'll most likely never even see it or know you're being tracked.
Even the "proper/official" ones are suspiciously low profile sometimes. We have one on my daughter's phone, she does know it's there and actually finds it quite convenient (e.g. don't need to ask where to pick her up if she's out with friends, just go to her on the map
) BUT you can turn off all notifications etc. for it and it runs in the background at all times... if you got that onto someone's phone and changed the icon to "Google Calculator" or something like that, most people would never know
.