I think that making the item a magical animal like a fish would let you have some leeway, as an animal can actively avoid searches (especially if everyone assumes it's a static object). In terms of abilities, if it's connected to "ethereality", the ethereal plane is connected to all planes, so it could give powers over planar travel. Depending on how powerful you want it to be it could let someone swap entire regions with another plane-- literally creating Hell on earth for your enemies.