Mystery vomit… apologies in advance!

Sorry I know this is a terrible topic for a first thread, apologies in advance and a warning I’ll describe it!! But just really intrigued..

We have two indoor cats who have a small run just outside the back door they access through a cat flap, and a back garden enclosed by 6 foot fences - it’s not cat proofed and we do occasionally have visitor cats - did notice one recently - but pretty sure no dog could get over.

We woke to vomit on the stone right outside the cat run, (far enough away that definitely not from ours) which was huge - nothing like we’ve ever seen any of our cats do. Sorry for the description but was solid and bigger than a tennis ball and full of small black gravel stones and also a few bone like a large fish bone.

Totally intrigued as to what could have done this - would be so surprised if a cat had eaten stones, and wondered if this could be a fox?

Have never seen any sign of one around before but very intrigued. Hoping whoever the culprit is they are ok and felt better for it!!