Apparently there isn't much xenon, from any source, in the Moon's "atmosphere".
I put "atmosphere" in quotation marks because it's actually an exosphere, where the distance an atom or molecule travels between collisions with other atoms or molecules is greater than the scale height. In the Moon's case, it is much greater. Overwhelmingly, particles (atoms or molecules) in the Moon's exosphere are on suborbital trajectories, from the point where they were kicked (or bounced) off the surface to the point they re-impact.
The best measurements of lunar exospheric composition were made using the NMS instrument on the LADEE lunar orbiting spacecraft. NMS is a Neutral Mass Spectrometer with a quadrupole mass analyzer, capable of measuring species up to 150 units atomic mass (I think "units" is the current term du jour; for a while it was AMU, "atomic mass units", then it was "Daltons", but Dalton must have done something non-PC because it appears that term has been abandoned). The 150 unit spec is fortunate because the xenon isotopes average out at ~131 units, so they're within LADEE-NMS's range.
However, the quoted abundances of elements detected in the Moon's exosphere don't include xenon. In the Wikipedia article they quote abundances down to 17 particles per cc (for potassium — not your usual atmospheric constituent!). They quote other noble gases: helium, 5,000 - 30,000 particles per cc; neon, up to 20,000; and argon, 20,000 - 100,000. But no xenon, or krypton. The helium, neon, and argon are delivered by the solar wind, and krypton and xenon should be also (sorry — paywall) ... so where are they?
My theory is that the sublimation temperatures of krypton and xenon are high enough to make a difference. If you look at the sublimation temperatures of the noble gases, the light ones (He, Ne, and Ar) are all below 100 K. Those of the heavier ones are above 100 K. If the same mechanism proposed for trapping water in the permanently shadowed regions of the lunar poles is cold enough to trap water on eon time scales, then it could trap the heavier nobles also.
Should we look to the lunar poles as a source of Kr and Xe for future lunar colonies? Maybe! ... if they need it.
As far as the behavior of Xe atoms expelled from ion engines, as stated in the comments they are travelling much faster than lunar escape velocity — bye bye, xenon atoms! Unless — they impact the surface.
The speeds of the ion-thruster xenon atoms are not all that different from solar wind speeds, so they would behave as did the atoms impacting the Genesis spacecraft's collector wafers. (I was on the Genesis flight operations team as the unofficial "Mission Generalist" — I did all kinds of multi-disciplinary calculations and analyses nobody else wanted to do) Those impacting atoms were imbedded into the crystal lattice of the wafers. They would do the same into the crystals of lunar surface materials. But when the crystals get warm, and on the moon they get very warm during the day, lattice thermal vibrations mobilize those imbedded atoms: they can move around, slowly, pretty much Brownian-motion style. Eventually they make it to the surface of the crystal and are ejected. If the ejecting surface is on the lunar surface and the atom leaves with an upward velocity component, then voila! it becomes an exospheric atom. That ejection energy would be far less than the implantation energy.
Such an atom could be bounced around the lunar surface for some time, maybe even at times re-imbedded (shallower than the original event) and re-ejected, until one of its bounces takes it to a permanently-shadowed region. And there it stays.
This loss mechanism could result in atmospheric Kr and Xe abundances far, far less than those of the lighter noble gases.
