Using mint 18.3 for ages, I returned home, switched on the laptop and instead of booting mint as it always does, on this occasion it went to the boot option screen. I didn't boot it, instead i switched off the computer then on again after a while to see if it did the same thing and it did, so i selected the system, (the only one there, also there where no peripherals plugged in) Since then it has gone to the boot option screen once in the past week.

Why would it have gone to the boot option screen out of the blue on 2 occasions since I've been using the system for many months?
I have not booted from a usb since i installed it
I have system password and bios password.
I live in a shared apartment so i thought maybe someone acceced it or tried to. But maybe it's just a system glitch.
I

I would try a disk check, and maybe a memory check, and then a system update & upgrade. Then worry.