WAL files containing old and unmodified data?

Hi all,

I have a very active database that generates between 200 and 400 WAL
files that are archived for backups. Yesterday, the average number of
WAL files being generated skyrocketed.

We took a look at some WAL files using the strings command, and notice
quite a bit of data from a specific table with timestamps of last year.
This table does some basic tracking, where a row is inserted, then it's
updated, and after that it's never modified again. So we're having data
appear in our WAL files from rows that haven't (or should not have been)
modified at all in over a year.

We figured that the rows were in fact being updated, so we turned
log_min_duration_statement to 0 for a period of time and monitored all
queries going through, however we never found a single query that could
possibly be updating these pieces of data.

Is there any other reason why a WAL file could contain data that (we
believe) hasn't changed? Or is the fact that it's in the WAL file a
guarantee that the data is either changed or new? Can server processes
such as autovacuum processes cause this behavior? Anywhere else I should
try to look?

Thanks in advance,
- Brian F