Earlier quoted context omitted.
> There are some parts of Matthew and Luke that neither come from Mark nor Q so they each also have one or more other sources unique to themselves. Given that we know nothing about the provenance or contents of 'Q' - it's lost to time, and its hypothetical existence is effectively intuited by some researches - I do not understand how we can identify the set of things 'not in Q' (or indeed 'in Q'). I'd also note that…
So your theory is they simply made up the exact same stuff? Maybe you do believe in miracles after all then.
Q is literally 'source' - with unknown author and unknown content - so we don't know what's in it nor what's not in it.
Therefore I challenged the claim by parent that 'anything not in Q ...' because that's an objectively ludicrous statement.
My 'theory', inasmuch as I have one, is that the authors of Luke and Matthew had different target audiences, different incentives, but predominantly the same (probably two sets of) materials.
They made different mistakes than the author of Mark, and indeed each other, though were presumably much more earnest.
The author of Mark was probably just writing allegorical fiction.
The author of Matthew for instance was the only one to talk about saints / zombies rising from the graveyards and walking into town, being seen by many people, etc - which is an odd thing for everyone else to have missed or not think to be worth mentioning.
There's a beautiful graphic showing % breakdowns of the Mark / Luke / Matthew content about half-way down this page: https://rationalwiki.org/wiki/Gospels
EDIT: that whole page is worth a read to get a better idea on the bleed between the different texts, a historical context for when they were probably written, and a reminder of the earliest copies we can reliably put dates to.