MacBook has 2 guest accounts, one with a password that I never set
If I purchased a open box computer, The first thing I would do is erase the entire internal drive and reinstall the operating system.

My preferred method, for erasing and reinstalling, is to use a flash drive. The instructions are given at How to create a bootable installer for macOS.

Once booted from the flash drive, I usually erase all previous partitions and create a single partition for the installation of macOS. This can be done by using Disk Utility application included on the flash drive. More information can be found at How to reinstall macOS. Look for the section "Decide whether to erase your startup disk".