Grinding pads but no warning?

[LunarGrape]

Hi all, over the last two weeks I have had grinding on the rears when braking and the last bit of slowing, it also starts that way as I pull off having released the brakes. 
 

I was going to just change the pads this weekend but when looking into it it says there’s a wear sensor, but I’ve had no warnings so am now doubting myself? 
 

Any thoughts?

[Raistlin]

Possibly a small stone between pad and disk?

[LunarGrape]

I’ve had a look and can see the inside pad is down to the metal I think but outer is ok. Also looks like the wear sensor was shorted out previously.