finally uncovered the mystery. Turns out one of the plug-ins I had installed had its own single-product.php. So I have two of them. Still trying to understand how they work together but I found the location of the continue-shopping button on my product page. Now I need to do the same with my cart, which also must be redefined by the gotoWP plug-in.
Need to see how to change the logic but keep it upgradable…
Thanks for the right directions.