OP forneceu feedback sobre a experiência:
Here's what happened. I went through the steps of 'cancelling' the 6-month vehicle permit, turning in papers and scraping the sticker off the window, as I was returning to the US. The cost was rolled back onto my card. At the US border, I explained to the officer that I couldn't find my passport. With my current driver's license, confirmed by my vehicle registration, and after the dog sniffed his approval, I was welcomed back with a friendly scolding to be more careful with my important papers!