.. to be more precise (as that may be the cause), the 4 pads on the upper row existed before (SWITCH1 and SWITCH2), assigned to two of the round pins.  I then changed which pins they connect to, removed their existing wires, and added the lower row of 4 pads (TOUCH1 and TOUCH2).  Now however DRC does not catch the unconnected nets, even though the UI does?

Your description would be much clearer with some before and after screenshots. Have you refreshed the nets in the DRC panel? Is this PCB created by conversion from an EasyEDA schematic?

Thanks Andy, I did not realise I had to manually refresh the nets first.  First, why is this needed at all?  I assumed the checks reflect the UI (which does realise there are unconnected nets). It also makes more sense to me that running a DRC check does that automatically, ie. that it actually checks the state of the current project as-is.  Otherwise this is error prone.