Aha! Upon investigation of some slides I sent back, the lab discovered that they were actually some sort of drying marks on the film, not scratches. They wiped off when rubbed with a cotton glove. They suspect it may have occurred on the occasion of dumping and renewing their chemistry, since they don't just keep replenishing forever.

I'm going to take them up on their offer of free replacement processing of my next rolls, and fully expect to be happier with the results!