The only evidence I have submitted to a half dozen employers of my right to work in the United States (because it's all I've ever been given) - is a cardboard I-94, filled out by me, no less, with a faint, blue stamp and date, and a (once again handwritten, this time by ICE agent) "Class: TN, Until: 07 Oct 2015". No Serial Number, no tracking number - just a stamp and visa class.
Not trying to scare you, just noting that I have had at least one friend on TN's who tried to bridge their status without filing a proper change in status ( ie. to B-2 as soon as you quit/are let go from your old employer, to be eligible to stay without packing up your apartment) and were banned from re-entry for 2 years.
I have had several TN's over the past 16 years and the only trouble I have had was primary border guard overreach once or twice (they felt I had been in the USA long enough, arbitrarily. Secondary review fixed that.)
I did not realize I could file a change in status if my old job concluded before I got a new one. In 2003, I finished up with a company, and my last, "Paid Day" as an employee was September 26th, but I wasn't required to come into the office for the last two weeks - so my last day in the office was around September 12th, a friday. By Sunday, September 14th, I had emptied my apartment, packed up all my possessions, and had returned to Canada.
Returning to Canada after an extended stay and then returning to the US, you will almost always be questioned.
Stamps are old school, they log your passport or NEXUS on each entry/exit in their system.