Almost any container is 'kind of' safe for a little while, such as if you just need to fast travel back-and-forth to drop stuff off quick (a few days), but you want actual safe containers for long-term storage. Basically you want containers that will never respawn (overwriting whatever you placed in them), but there's no real way to see which containers are safe or unsafe within the game. If you look under 'Incidental housing' in that link you can find a list of places with 'safe storage'.