DM's call, but most likely "Firsthand"
Scrying tells you how it measures familiarity:
| Knowledge |
Save Modifier |
| Secondhand (you have heard of the target) |
+5 |
| Firsthand (you have met the target) |
0 |
| Familiar (you know the target well) |
-5 |
"You have seen the target" is not on the list, so the DM will have to make a call what it is most similar to.
Firsthand fits best. In the absence of defined game terms, we use common English usage. Firsthand is defined in the dictionary as
coming from the original source or personal experience; gained or learned directly.
The ellipsis gives an example of something that would count as firsthand, a personal impression of the target ("you have met the target"). In my opinion, having seen and heard the target through Scrying once or twice is not that different from having seen them in person. You do not have to touch someone when you meet them, nor be so close as to smell them either, so how you get to know them in this way is functionally quite similar. The important part is the first-hand experience.
Second-hand fits much less. It is defined as
(of information or experience) accepted on another's authority and not from original investigation.
This describes the case where you have no first-hand experience of the target, but only heard about them or have descriptions of them through others. Seeing them personally (even through a spell) is more direct than just having heard "of" them. You have seen them through original investigation with the spell.
Familiar: Lastly, if the caster has been scrying on the group for an extended period of time, one could even make a case that this gets into "you know the target well" territory: they get to know its behaviours, mannerisms, way of thinking -- that is what one could count as knowing someone well. (Familiar is also not exactly defined in the game, see this Q&A, but here it clearly must be knowing them better than from a single meeting. Other spells set the bar lower.)
They certainly would be able to scry on you, even if you blocked their connection to the Kenku. Scrying even works on creatures that you only heard of.
As a side note: if the other party scrying on you were your enemies, why should they use Sending to warn your rogue about a dangerous ambush? It might be worth trying to check the assumption that they are indeed your enemies. They might be on your side, too.
