Resetting NVRAM (aka zapping PRAM) disables Find My Mac! This is a remarkable flaw in Find My Mac. You must set a firmware password to avoid this flaw.
Find My Mac is a potentially useful way to locate or erase your Mac if it is stolen. But this flaw makes the feature essentially useless if a thief knows about the simple trick.
If you want to use Find My Mac, you must set a firmware password to prevent this flaw from being used. Setting a firmware password prevents zapping PRAM without the password.
See the following Apple Support articles for more information:
Make sure to read the entire Apple Support page about the firmware password so that you understand the changes on the Mac when you set a firmware password.
This flaw makes it clear that the setting for Find My Mac is stored in nonvolatile RAM. It would seem that a simple workaround for Apple to add would be to also store the setting on disk and when the two settings do not match, warn the owner of the Mac that the settings need to be checked.
This flaw continues to exist in Mac OS X El Capitan 10.11.5.
Tip: Don’t forget that WiFi must be turned on for computer to report its location to Find My Mac.