Could my stolen Mac have been erased before I activated Find My Mac?
If you don't have configured a password to choose boot options the thief is able to delete all your content, without having your root password.
He can simply:


Do not connect your Mac in internet.
Turn on Mac pressing Cmd+S to enter in CLI mode.
Change your admin account password (I have intentionally omitted details on this step).
Logout.
Login using your account with his password.
Disable all services related to iCloud, so disable Find My Mac.


In this case you're unable to locate your Mac.