Even without a cap, gravity "should" prevent the drain leaking - the connection to the hose is some height above where it joins the U-trap. If the drain backs

Even without a cap, gravity "should" prevent the drain leaking - the connection to the hose is some height above where it joins the U-trap. If the drain backs up and the pressure is high enough to force water out, it might be enough to pop any screwed cap too.