radar updates happen every thirty minutes, meaning the UFO can move well inside your detection radius at 15 minutes past the hour, and he won't be picked up on hyperwave until 30 minutes past. likewise, detected UFOs leaving your detection radius won't become "undetected" until the half hourly interval.
as you can see in the first picture, the time is 15:29:10. the UFO which is outside detection range will "disappear" in exactly 50 seconds., same with the second picture, you're 5 seconds past the half hour mark, meaning the alien flew into range at around the 25 minute mark.
the radar only "pings" on the half hour, and so it was detected THERE.