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Alien Comet May Have Infiltrated the Solar System
New Scientist has a piece about Comet Machholz 1, whose uncommon molecular composition suggests, but does not prove, that it may be an interloper from another star system. "Comet Machholz 1 isn't like other comets. David Schleicher of the Lowell Observatory in Flagstaff, Arizona, measured the chemical makeup of 150 comets, and found that they all had similar levels of the chemical cyanogen (CN) except for Machholz 1, which has less than 1.5% of the normal level. Along with some other comets, it is also low on the molecules carbon-2 and carbon-3."
Lets face it, even if we retrieved a sample and analysed it in a lab, we wouldn't be able to say with any real certainty where it came from.
With an actual sample, and isotope analysis, we could say whether or not it came from this solar system. This is done all of the time for tiny grains found in meteors and collected directly, some of which do not come from this solar system.
True, saying where it did come from might require sampling most of the star systems in our region of the galaxy and that will take... a while.