Claiming residency retroactively

I have been living & working in the US for the past 12 years on TN status, but when I left Canada, the internet was not so available and I did not even have a computer. So, I never submitted any form to any Canadian agency to say that I was leaving. I never knew that I was supposed to.
There is no such thing as an 'exit' interview with Canadian officials.

What will I have to deal with upon my return to Canada? Obviously I don't want to pay Canadian taxes on top of the US taxes I paid for the past 12 years.

In Canada, I do not own property, had a canadian bank account with nothing in it which I closed after a year or 2, same with a canadian credit card, and did not use OHIP (Ontario health insurance) at all for the past 12 years.
Any insight / help / advice would be greatly appreciated.
Thank you!

Sounds to me like you are in the clear. The current interpretation of this is that if you have been working out of Canada for over 6 months with no intention to return to Canada, then you should be in the clear. If you have been away for over 5 years regardless of your intention to return to Canada, you should be in the clear also. Of course "intention" is a very subjective term. The main thing they can use against you is if you had a residence in Canada that was available for you to move in. So if you were renting your house out, then it was not your residence anyome, it is an business. Other things they may want to use is whether you had a bank account, drivers license, health card, etc. However, those are secondary, and they probably will not pursue you over those.

As for proving that you were not a resident in Canada, the TN-1 stamps in your passports should be more than enough proof. You probably had rent leases in the US also, those will be more than enough. Since you did not work in Canada during that period, they probably do not have any reported earnings for you.

It is extremely unlikely that they will even bring up the previous 12 years of your being away, when no one has reported paying you money in Canada. If they do bring it up, just tell them that you were out of the country the last 12 years, and that should be the end of that.

You just need to give the CRA a call and tell them you're non-resident and forgot to tell them 12 years ago. They might ask you to fill in an NR-73 but other than the waste of time that will be, you won't owe any back taxes. The only slightly trivial problem you may have is if you failed to file a T1 for the year you left or filed it for the full year but you left mid-year and should have pro-rated it.