Phone volume controls dissappearing

I've a Nexus 5, upgraded to Lollipop a few weeks back. When the phone is turned on, the volume controls work as expected for a time. After a while though they stop controlling the volume and on the screen appears a little Chromecast icon with a slider. This looks similar to the volume control, but sliding it left and right has no affect on the phones volume. I'm not using Chromecast, and there isn't one on the networks I use, so I've no idea what it's controlling. Alas, the only way I've found to reset the volume function is to reset the phone.

Perhaps this is a bug? If you think it is, can or should I report it somewhere? And perhaps unrelated, I seem to loose mobile network connection completely after a while also. Again only way to fix is a reset.