Hey, I am happy to hear, that you liked the game!
In regards to pirate raids, there are auctions, those should have an icon and sound notification, but of course one can miss those, if they happen during an active battle.
Otherwise, the mechanic works as this:
The lowest bidder gets raided (so if you did not bid, you will be raided. And if you and your friend both did not bid, both of you will be raided).
And the highest bidder gets a one time ability to send a raid, he can use at any time.