Np.
Anyhow TPCi themselves aren't clear with what they mean altered data. I'm assuming it's just a catch all term for any kind of hacked data, whether is it (and not limited to) save hacking or modified mons.
Well, when it comes to detection, the whole point of legal mons, is that is it indistinguishable in terms of data.
However, all of that operates on our presumption of a legal mon. We constantly update our legality checks.
So it can result in situations where a hypothetical mon A was illegal, but it was hard to catch to us (for various reasons, such as we did not create the game and may have missed some super technical explanation, such as memories, for example), and was thus flagged legal.
Anyhow, it really depends if they bother with such an in-depth check and catching things non-Ninty/non-GF devs missed.
(Tho given our other extensive checks, I'll say it's pretty unlikely for an illegal mon to be flagged legal nowadays)
Next, just because your save is filled with legal mons doesn't mean it cannot be detected that a save was modified. Maybe the save other edits, such as an impossible amount items between 2 save states (stored in the logs on their servers) with very little advancement time-wise between both save logs. Or maybe it compare 2 save states and see there's different mons in both saves, despite no changes in withdraw/deposit/trading values. Anyhow, there's probably lots of ways they can try to detect altering of save data, but it goes back to my previous reply:
Anyhow as long as one reckless do save edits, there's probably something said person will miss or do wrong.
Just because you got legal mons don't mean your save is 100% safe.