Ran scrub on all my disks. The only errors were on this same disk: 5 corruption errors in total (across 3 files). Also, extended SMART on the disk came back clean (reallocated, pending, uncorrectable all 0).
The 3 files were all modified a year ago within hours of each other and inode numbers are just 10 apart or so. On the other hand, the single UDMA CRC error appeared 6 months ago, so even more likely to be unrelated per your comment.
Do you think this was non-hardware BTRFS corruption during writing or afterwards? Or some physical bitrot that didn't show up on SMART? Both would be pretty rare, but can't think of other explanations, especially given this a single disk BTRFS, no ECC errors in BIOS log, no SMART extended test errors.
[edit] actually physical bitrot should have resulted in a CRC error from the SMART test, so probably some random BTRFS software corruption