I was on a TN for 13 years (I'm on a H1B now).

The rule for the TN is if you travel outside Canada / Mexico then you are to surrender your I-94. I once asked if I leave from Canada to Europe am I to surrender my I-94 and I was told yes (I've never done so).

When you enter the US again, they will have all your TN Visa details and they will issue a TN for you without having to send you to the back for secondary. Normally what I would do is have my I-94 already filled out so that he can re issue it. This was the case all the time I flew from Europe to IAD and then on to FLL where I was working / living.

Same thing with the H1B. I came back from Europe through ORD and they issued another I-94 at the counter.

Since I have a NEXUS card and I don't want to keep going back to update my I-94 information on my NEXUS profile (since this can only be done in person) I just take the I-94 form out of my passport and put it back once I get back home to YVR and enter in the US on a C1 visa (in transit). I only do this when I'm connecting back to YVR through the US. If was going to stay in the US from Europe, I would then have another I-94 form ready. This was the advice given to me by the NEXUS office by US immigration when I asked about whether I can call in my new I-94 number. They said, it has to be done in person and then she said just take it out of your passport.

I hope this helps.
Cheers