As indicated, they are in the same vlan. No routing involved. Thanks for helping explain the airgroup more.
While waiting for a reply, I did find that when I issued `no bcmc-optimization` in the vlan interface in question, suddenly I am flooding packets everywhere and I can see my Chromecasts.
So the next question is going to be, is there a way to get these particular things to work without turning that off, or am I best to just get creative with some acls at this point to reduce flooding of multicast and broadcast everywhere?