If anyone had even noticed.  It was hard to say how many instruments there were on earth peering at the sky.  Of those that were, how many of them were set up to measure the exhaust of a spaceship accelerating to warp speed?  Even asteroids often got missed until they were fairly close—and those were only the large ones.  Unless the ship was miles long, it was a very tiny object in a very vast space; it might have only been seen if a telescope coincidentally happened to be looking right at it as it went by.