Regarding lawn mowing as a potential cause:
I wasn't home at the time. The two closest neighbours weren't mowing their lawns at the time (one done earlier in the week, other wasn't home at the time). Maybe one diagonally across the street but I couldn't hear any mowing on the outside camera audio.
I did consider mowing as a possible culprit but it would have to have cleared a 1.2m fence in the process. But yep, stones from a mower can certainly fly a long way.
Will probably never find out where it came from.