But you weren’t stopped or anything right? Just extra questioning? Also, what other documents did you present at immigration when coming back other than your RTD?
Probably we will always be sent to secondary until becoming citizen (some people even after that).
Don't bother going to primary kiosk machines. They will print an invalid receipt with RTD, just go directly to the office desk.
First official asks the regular questions.
Where did you go, how many days, do you know someone there? Reason of the trip? Bringing money? Tabacco etc etc?
Handle you the paper with some lines with a face like they hate you, their job and the world.
Next officer in the hall will tell you to go to the secondary and turn left.
Same previous questions and additional ones:
Did you go another country besides this? Did you go alone? Is this your first travel leaving the country? Why did you go this city? Let me see your boarding pass.
Gives you the paper back stamped.
And the last officer will say if you need your luggage inspected or not.
Tip: When going to Mexico they don't automatically stamp your passport with exit stamp, go to "oficina de migraciones" and ask an exit stamp. They are before the security screening and it will make your life easier when returning Canada.