MacBook has 2 guest accounts, one with a password that I never set

My MacBook Air that I've owned for just days has its own guest account, with its own password. This happened to me once before back in 2013 on a MacBook Pro. Typically, I disable the guest account. However, I recently purchased an open-box MacBook Air from Best Buy. I noticed that sometimes, but not every time, a pixelated Guest account would appear next to my user account when logging in, as seen below:

Now most of you might tell me that if I have iCloud & Find My Mac enabled, then the Guest account is there by default. This I'm perfectly aware of, and when I enable Find My Mac, this is what I get:

Once again, we have a pixelated guest account which is password protected, and then the guest account enabled with Find My Mac (on the far right) which doesn't require a password.

What is going on here?

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".