Usually on start up, but can happen at any time, the game will warn me that something of mine is floating away. The item in question is usually my raft, but has been any number of other things. Most recently it was my buoy balls, and once a hammerhead shark.
In all cases, the items in question, including the raft, where fully on the beach of whatever island I happened to be on at the time, except the buoy balls. Those where in one of the crates on my raft and in no way at risk of floating away.
The game also likes to tell me one of the items I have in my bag has been added to the bag right after loading up game. Sometimes before sometimes after the warning of the floating away item/raft.
No custom islands
I mostly notice it after load-up, but I have noticed it during normal game play.