Copied some personal personal files off my virtual desktop and computer.
First major red flag. You were not an employee of the company at that point. Therefore, accessing their systems was an unauthorized intrusion. Just because they didn't remove your access, doesn't mean you're allowed to be there.
It would be like my landlord evicting me, and then me going back into the house afterwards because he hadn't changed the locks yet.
On Monday my supervisor called me and told me I had to return the phone and computer. This surprised me as it ran contrary to what I had been told before but I didn't feel like arguing about it.
Did you have it in writing that those items were yours? A reasonable person would assume that the company paid for them, so they are company items. Implications by your (now former) boss don't cut it. So the fact that you had to be contacted to return them will work against you.
Last night I get a call from my supervisors that he's getting weird question about what I was doing on the device after I was let go. He's being very vague about what exactly why questions are being asked or exactly what I am being accused of.
He's trying to get you to admit you were illegally accessing their network. An argument could very well be made that you were, because:
I apparently signed in multiple times over the weekend. Not surprising since it would automatically connect to company servers every time which I would simply immediately log out. I moved my files some personal files from both the hard drive and my virtual desktop.
And you just confessed (again) to an illegal intrusion. Them not being super secure and revoking your (previous) access, does not mean you can take advantage of it.
Today he's trying to reassure that he smoothed thing over but insisted that I send in a note of apology for accessing those servers. He's convinced this is going to help me. He is dangling a possible small severance package for doing this.
He's trying to get you to hang yourself. If he has it in writing, it's a slam dunk for him to take to law enforcement and his bosses to negate his own culpability (for not securing the network by removing your access) and string you out for committing a crime.
OP, I would highly, highly recommend getting an actual attorney and deleting this post, because you're essentially admitting to a federal crime that is punishable by up to ten years in prison.