If aliens have probed us, I DOUBT we would have actually noticed them. Because, likely the probe would be about the size of one of our satellites and maybe even a lot smaller to cut down on the amount of energy needed to get them here.
Sorry if I take "explosion makes rock go far" as being the more plausible explanation than "Aliens want to wait a million years for a brief glimpse at a distant solar system".
Of course, that is still less likely than some unusual rock doing the same thing.
Playing Devil's advocate, a probe that size would likely visit many star systems, the Solar being a random one in the bunch. The brief pass would be necessary to avoid the energy expenditure of braking and reaccelerating.
1) its shape may actually be more like a pancake than a cigar, a crumpled light sail would match this 2) it showed unexpected acceleration away from the Sun without outgassing. 3) its velocity is very close to the mean motion of our local stellar neighborhood, if you needed to drop a solar sail after deceleration so that it would be hard to work out what system you came from then this would be the velocity to do it at.