Update on this, as I know the experienced members here like to hear outcomes. Even though this is a partial one.
Papalon met me at my office, and both bikes side by side the issue is obvious. The front right brake pads have worn quicker than the left, though they have plenty of life left in them. I also did a quick roadside check of the SMC/PCV, no issues evident there. When lifting the front tyre & spinning by hand, there's an audible difference between left & right calipers (also more noise than my bike makes).
A 5min ride in stop/go traffic with him following me confirmed that the right disc warms quicker than the left, conclusion is a binding right caliper.
I've suggested he books the bike in for a strip & clean of the calipers. While in truth it's a simply DIY job, if the bike is under warranty & there's a chance of getting it done for free... that's the route I'd take.
I'd wager that pulling the pads & doing the shoelace trick on the right caliper pistons would solve it. It doesn't appear to be as bad as an over extended piston warping the seals etc. Probably no need to change fluid (though I didn't check, but the bike does have recent service history including fluids).
All in, less than 30mins to diagnose without any tools... not sure why a garage couldn't ID it, though it is slight, so possible they overlooked it without a test ride.
He also spotted some farkles on my bike that he liked...
Finally,
@papalon - as I'm sure you've seen in PM now, my apologies, I gave you the wrong phone number originally, that's why you couldn't get in touch.
It's correct on the RAN list though.