(EDIT: Neutral AI bringing...)
I am playing 2.3 as Rome and having a conflict with Areuakoi in Iberia. I have no claim to Lusitanian lands and have proposed alliance with them, which they rejected a couple of times. I have tried to steer clear of their areas as well not pose an immediate threat, as I do not want another enemy.
The Lusitanians, however, have started doing three things and have kept at it. First of all, they bring a huge navy (about ten stacks of two ships each) to the Balearic Islands, which I control. The next turn all of them leave only to come back again. They do not attack any of my small navies operating in the area as troop transports. Second, they bring a single unit there next to my settlement (disembarking on land), but do not lay siege. They stay a few turns and then go away, only to return again. Third, they keep an assassin next to one of my continental Iberian settlements but never attempt any foul play.
Are they on the verge of attacking me but then just each time decide it is not worth it? The relations are not worsening each turn anymore, although they were for some time earlier.