After a little more digging - the ground radar system apparently turns targets yellow when there is a potential runway occupancy conflict, but there is no aural alarm. That’s a problem in a tower, because the tower controller’s job is mainly to look out the window, not stare at the radar. We had the same issue in the US about 25 years ago, and the NTSB recommended the FAA add aural alarms to tower displays to attract attention to the display when necessary. Sounds like something the Japanese will be doing soon, too…although I’m a little surprised they didn’t have it already.