phantom long-running query (check_postgres.pl reports a long-running query, but the database logs don't show it)

I got an alert from check_postgres.pl today on a long-running query on
our production database, but our PostgreSQL 8.4.9 server log, which is
configured to log queries over 5 seconds long
("log_min_duration_statement = 5000") does not show the query.

check_postgres.pl showed:
Date/Time: Wed Feb 8 08:41:36 PST 2012
POSTGRES_QUERY_TIME WARNING: (host:xxx) longest query: 264s
(database:xxx PID:xxx port:xxx address:xxx username:xxx)

postgres log showed other long-running queries, but nothing over 48s.

It's really interesting why the database server log does not show this query.

Can you think of some scenario where check_postgres.pl would see a
long-running query but it would not get logged by the database server?

I checked the server log and there is nothing there for 08:41 at all.
08:40 and 08:42, yes, some queries over 5 secs, but nothing even close
to 264 secs.

I've added the verbose switch to my check_postgres.pl script so we'll
have a record of what was the long-running query.

Best,
-at