I think they would have caught him, being that close to the incident. Maybe another of his powers?
It was played up for flaire. In reality he probably got out of dodge as soon as he heard the siren in the distance.
And as he said, the property was big enough to pass of as a park, with plenty of places for him to dodge through including tree coverage, especially with the general "HOLY SHIT HOLY SHIT HOLY SHIT" freak out everyone is going through, so far far from impossible to just sneak away as "HOLY FUCK THE TERRORIST JUST LAUNCHED A FIRE TRUCK AT THIS MANSION, THE WHOLE THING IS ON FIRE AND LOOK AT ALL THOSE COP CARS!" Classic bait and switch