If a TN applicant returns to Canada before the TN Visa period ends and no longer is using their TN Visa, US Immigration needs to update their files for date of departure. Otherwise upon returning to the U.S. at a later date could raise questions. If one flew out of the country, it is more evident that one left the country. However, if one drives across the border there is no official record departure.
Take the I-94 card with you on your next entry into the U.S. but also have supporting documentation showing when you left the country previously. Use this article: Forgot to Return TN Visa I-94 as a guide to assist you.