Magic player detectors are bought in Marlybone for 1300 gold from Babbage Basset sometimes you can find them in the bazaar for a lower price. Don't forget your spells or animation cards you will need. I bought the scarab and invisibility for this topic.
Set your items down. I wanted this area blocked off and only when a player crosses over the detector will it open. Make sure your detector is right over where a player would walk. You don't want a player having to find when a detector is after all you can't see them.
Now click on the orange detector and set your invisibility spell and set it to the item you want to go invisible. I'm making the Kroktopian statue go invisible. Now set up a spell for when a player leaves I use a target item that players can't see like bread crumbs or other castle magic pieces.