And if so, will I be able to register for their on-going promotion right now for free mags?

Just looking for personal experiences or opinions - don't expect any legal advice so feel free to comment.

How do you know that the gun wasn't registered?

All original materials in the box; first owner used for a week and traded in; said never registered.

I have registered quite a few used Sigs on the factory site. I do it so I can be notified if Sig decides to recall that model or serial number range.
Hasn't happened yet.
Don't know if you can score any freebies by registering though.

Back in 2015, I registered my P320C 9mm on-line. Still have registration card in box.

At the time, if you registered on-line you could get 20% discounts on good stuff at the SigStore. That's how I got my P320FS .40 X-Change kit for $240. List was $299 back then.

Given the history of your P320, I wouldn't feel guilty about registering it. It's not like a two-year old gun with 10K rounds through it. Other than registration, I don't know how Sig would track original owner versus subsequent.

Can still register on-line, but no discounts anymore.

https://www.sigsauer.com/support/warranty/

Actually right. now they have an offer for mags and a bag.