I've exited the US for a short (few days) trip to Canada. I've used my passport, but forgot to take my old passport — which has my (unexpired) US visa (tourist).
My current passport carries my most recent US entry stamp, and my I-94 is still valid.
Is there any chance they'll let me re-enter based on that? I know one can re-enter with an expired visa, but this case is different: the visa is unexpired, I just don't have it physically with me.
You would have been able to enter without your old passport. Given that you spent less than 30 days in Canada (this would have been true for Mexico as well) your visa is re-validated upon entry (automatic revalidation).
Automatic revalidation does not require you to present a visa - it is the I-94 that is required. If you originally entered the US by air or sea, your valid passport is enough (the I-94 is electronic in this case), and if you originally entered the US by land, you present your passport and paper I-94 form (which should have been stapled into the passport).
Source: https://www.cbp.gov/sites/default/files/documents/auto_reva_3.pdf
No, you will need to show your visa for entry into US. This visa can be on your old passport too. However, you need to show your visa.
