My site uses an (admittedly) old theme called Blox (http://www.themezilla.com/themes/blox/). Twice in the last few days the site has been changed completely when a different theme called Blox has been installed in it's place (https://cyberchimps.com/blox-free/). I've now turned off theme updates in Automate in the hope of stopping this happening again as I'm pretty sure that's where the issue came from. So I should be sorted for now but I thought it was worth mentioning in case there's anything that can be done to stop this kind of thing happening in the future.
We're about to update the site to a newer theme so it's not a big issue on this one but could be something that hits others.