If actual/potential loss of life were minimal (as in the OP?) and Russia responds quickly with apology and remedial action, it could (would almost certainly) be handled diplomatically the first time. If no apology or discernible remedial action, then like for like is . . . likely. US/NATO would not attack targets in Russia (too much potential for propaganda), but would use it as an excuse to teach Russia (some of) the difference between our intel/targeting ability and the Russian ability - basically a punitive strike on Russian forces in Ukraine with intent of minimal loss of life, but maximal effect on the chosen target's war fighting ability. If the unit responsible for the NATO casualties was identified, it would be a likely candidate to be destroyed.
The above is from US/NATO operational "contingencies" planning.
The difference in US/NATO vs Russian intel/targeting capability for such an action is so bizarrely great that I find it difficult to effectively lay out.