We crossed back into Canada yesterday at the Ambassador bridge in Detroit. When we arrived at the kiosk the window remained closed for a half minute, the agent opened it and then asked us for our truck license plate number (we have a 5th wheel RV with bicycles obscuring the RV plate), the cameras only seem to look at the rear plates. Maybe that was the reason for the delay, he couldn't get a plate read on his computer.
The rest of the stop went well, he asked about our stay in the US, particularly Tucson where his father spends the winter. No other questions, we were on our way in a few minutes!