In HP 1 movie, Hagrid used magic to lit the fire and add a tail to Dudley. Now throughout the books and movies, it is mentioned that the can detect magic being used by anyone. EXAMPLES: Harry being the only in the Dursley's house they knew (well assummed) Harry performed magic. In HP & HBP the knew Mofin cast magic on Muggles. SO ---- Why could they not know Hagrid performed magic in front of Muggles in the first book & movie.