Sevigny is like one of those too-clever suspect/witness people that used to show up in Columbo episodes... having some clever inside angle and sidling up to the investigating detective all the time. As to what it means to the big picture... right now, your guess is as good as mine.
But poor Stills has some work to do on his surveillance... he thought the intruder was 30 to 35 years old. He's only off by 10 years.