Grub did not detect your VM, it detected a bootable operating system on the drive because you passed it through to your VM

Yeah, the bootable drive that contained my VM install, that’s what I’m saying.

But i prefer using a raw disk file image

I started that way, but I had a disk with a single partition that contained a single file - the raw disk image file, and eventually decided this is silly, the filesystem on that disk is useless.

I did that, and since I got a dedicated SSD drive for it, I used it for the VM as a block device. Later after a GRUB update I discovered Windows in my GRUB boot menu. Turns out GRUB detected my VM, and now I can physically boot into my VM. Which I didn’t even know was possible.

So yeah, I accidentally dual boot Windows without meaning to, even though it’s a VM. Except when I boot into it, then it’s not, apparently.