Can I cap off and ignore red wire?
Find the switch, before somebody else does and spoils your plans!

The problem with this plan is that someone will find the other switch, flip it to turn the light on or off, and inadvertently disable your now-single-pole light switch in the process.  Either that, or your single pole switch won't work to begin with because the "hidden" 3 way switch is flipped to the wrong position, energizing the capped off red traveller instead of the connected black traveller.  Moral of the story: find all the switches on a circuit before doing anything to it.