Errors and logs

In the my.cnf file, under [mysqld] I have log-error=/path/to/error log
file directive. The last entries in this file are over an year ago. In
case of MYSQL errors, such as improper roll backs or improper commits,
shouldn't the errors be logged here. And if this is not the right
place, where would the errors be logged? The users claim some data in
the database changed automatically, I want to investigate any errors,
but I don't see a place where that information is. Please help.
Thanks,
yip