One of the reasons it escaped detection in the initial captures (which have now been reassessed and backtracked) is that its orbital parameters were a bit above the expected range (eccentricity > 6 for example). Since we detect moving objects by either synthetic tracking (stacking images in order to increase SNR but doing all stacks in all directions and for a set of possible speeds) or variations of blinking (detecting differences in images taken with some delay), we are heavily dependent on the choice of parameters as they limit the search space. The parameters of many detection pipelines are now adapted to allow for the detection of more peculiar movements. There's still a computational limit on what we can detect though as the search space increases exponentially when parameters increase.
Now, to keep the spirit of this thread, even if it is a comet, shows the expected spectrographic features of a comet, seems to have a tail, we can not prove that there is no governmental conspiracy hiding the fact that it is a very advanced ship, masquerading as a comet, piloted by demonic entities, originating from an alternate timeline in an alternate universe.