Can an unknown clan declare war on you?

Just abandoned a legendary Uesugi campaign as even though it is barely into the 1550s half the map is already at war with me and two full stack sea invasions from clans I never even shared a border are about to descend on my capital.

As my previous Uesugi campaign had the same result (thanks initially to the vassal you start off with in Kozuke which was attacked by every clan in the east - and as I was roleplaying Uesugi Kenshin as honourable I couldn't just abandon them to their fate...) I am contemplating a radically different strategy for my third attempt

This will be radically isolationist: no attempts to corner trade nodes, no ships other than those required to prevent blockades of whatever ports I do get trade through, no vassals or alliances, focus on building up farms and taxes.

Thus the question - while I've played more hundreds of hours than I care to admit I can't recall if a clan that hasn't been discovered yet has ever declared war on me (as opposed to it declaring war on an ally).

Imagawa

Never seen it happen. Even Imagawa starts shipping full stacks to me after I meet them.

The AIs love to launch sea invasions after they know you, though. I think it is still OK to explore on the north-western shore as Uesugi, as you can still line up for the trade nodes, and your enemies don't have navies there. But sooner or later someone will ship stacks to you.