The Data Dashboard for Heroku Postgres instances shows commit activity, under the I/O section, even when the database is not being used.
This is expected behaviour. Postgres treats each executed SQL statement as being implicitly wrapped in a transaction, so
SELECT 1; is treated as
BEGIN; SELECT 1; COMMIT;. As a result, each time a statement is executed successfully
xact_commit is incremented in
pg_stat_database, which is what we use to determine the commit activity. As our monitoring tooling executes queries to check the health of your database, there will be a baseline level of commits being recorded.