As a Law Enforcement offier here in Southern California, an open container is one that is unsealed, decorked, unscrewed, etc. It is technically illegal to have it anywhere in the passenger compartment. To be safe, all alcohol, open or not should be in the trunk or cargo compartment.
The only time an open container or drinking or an alcohol beverage is allowed is when there is an active partition between the driver and those drinking (i.e Limos.
I have pulled people over before and seen open containers. Some I advise and some get cited. It all depends on the circumstances and their honesty.