As the title says really, noticed this blemish on the drivers side rear arch and since its right in known rust zone, I'm hoping it's not but would like a more experienced opinion.

As always any advice is greatly appreciated

They normaly go frrom the inside out, so first sign is bubbles under the paint.

Massive thanks Tidgy, that's a relief.

Do you think its a good idea to compound it and see if its on top of the paint? Or is it more likely it's worn clear coat and paint? To touch i feels smooth but don't want to make it worse. 

looks more like somethings scuffed it going by the pic but im no expert on paintwork by any means

I would try a mild compound just to see but agree with Tidgy looks like a scuff

Check behind the boot liners inside the boot and under the rear arch where the rear bumper fixings are . They're normally the first places you see rust but if caught early , you can hold it back 😉