It is often worth asking these “unofficial”/“unannounced”/“unintentional”/“unplanned” (however it can be described!) activations a few questions about their working conditions to establish whether or not it is a valid SOTA operation. I always do if working a 2m FM operator on Winter Hill G/SP-010 or Great Orme GW/NW-070 for instance (and some others). These are both possible “drive to the top”, so always worth checking.
Of course, if they are operating genuinely portable, away from cars, buildings and generators, then you can tell them that they are SOTA qualifying and advise them of the reference. Also, that they can enter the activation on the SOTA Database upon their return (pre-registration is not necessary).
So it can be a way to recruit a new op into SOTA.