The best way to protect your Mac’s data is to use FileVault.  Without FileVault, anybody who has physical access to your Mac can become root as will by holding down the command key and s key on the keyboard right after a reboot.  As root, that specific person can take a look at your data at will and might do something even more nefarious.  So, the next time, don’t think that user accounts on your Mac can be secure, because these accounts have passwords!  The passwords can only protect users from remote hacking, somewhat!