I took my car to Santa Pod to get my Toyosport exhaust system fitted. When they had the car on the ramp, I noticed the same thing. My car has done 44k miles so it's not far from your mileage. Maybe it's common?
Mine is also a UK car so it must be...
I thought UK spec'd Zed's were rust proofed?
The surface rust you have found is usually caused by the heat radiation from the OEM backbox! Not noticed any on my Zed yet but have seen it loads on all my old cars! (Focus ST170, Stilo Abarth, Citroen AXGT, etc...)I'd imagine it'll be fine as I've never heard of it causing problems!