Hi Ben,
Welcome to the forum and great that you have obviously been looking around or using the "search" facility already.
There is SO much information on this site.
Sadly, we never did get to the bottom of the mystery, which I think is more likely insect related.
However, after washing off with a "firmish" spray of water in a remote corner of the garden over the space of a couple of days.... I never saw any more of them. Thankfully they have not returned since.
Try the same now with that elm and keep just moist in a shady/sheltered spot for another good 2 weeks to help it recover from the loss of moisture while you were away.
It could most likely have been the high winds we've been getting of late.... since small pots will dry out in no time at all, even if your friend was watering on a daily basis.
Hopefully, you have caught it in time.
All the best,
Ian.