Thanks for that, and for the PM from another forum member who suggested the same thing; the plastic bottle has it, in my view.
I'm just back from a hundred mile round trip to check the C30 during daylight and then drive it home, leaving her with my V70 for tonight.
There is absolutely nothing wrong with the C30; everything is as it should be. Phew. Must be a bottle or plastic bag, now blown away (no debris visible on or near the road at the location where she said it happened).
2.4TSE is now relaxed again!
Thanks for taking the time to post and to PM.