By releasing the tidbit about the fire, I think LE is sending a message to TH to let her know LE is not leaving any stone unturned, and Kyron's disappearance is not going to just blow over.
LE might have reason to believe the garage fire was an attempted homicide set up to look like an accident, one of those situations where the intended victim makes a narrow escape. For instance, the garage door has been rigged to stay closed, but the victim manages to push it open just in the nick of time. Or he's on his way to the garage but stops to tie his shoe and pet the dog, and the fire starts before he gets inside. I know...sounds like a Lifetime movie. lol There must have been some strange circumstances if LE can look back on a 20 year-old fire and suspect arson.
If they've dug back that far and suspect TH committed arson in the garage fire, it's likely they've uncovered other criminal deviant activities in her past. None of it might pertain directly to Kyron, but establishing a pattern of behavior could help to determine what she did with to him.
How horrible it must've been for Desiree to spot her driving along the highway.