On linux, the apache error log is found in dir /var/log/httpd/.
I'm using Linux (Slackware) and the (main) error log is in /var/log/apache2/error_log.
The sysadmin can put that directory where ever he/she wants.
On the other hand, foreach site I always create a dedicated /logs dir under the site directory (not a public directory). This way I don't fill up the main error_log and makes it easier to follow errors on the web site I'm currently working.
I do this when I configure a new site on Apache: