Zap!

Reggie has been watching the City Museum all day.  As night falls an [oddly shaped] crescent moon hangs low in the sky, illuminating the landscape.

Though he is fairly certain, he wants to make sure that a trap has actually been set.  Reggie knows that they LCPD wouldn't be as obvious enough to set a visible trap.  Thus, he knows there as an invisible trap.  As he hides in the brush near the City Museum, he looks around for a way to test his hypothesis.

Reggie finds a hedgehog in the brush and throws it at the door to the museum.  Zap!  Just as he thought.  Once-invisible lasers light up as the hedgehog passes over the threshold.