Contributing back to Open Source doesn't always mean "in code". In fact, most of the open-source coding I've done were of the local customizations variety -- nothing that you'd really want to release anyhow
But "contributing back" can simply mean that when you hang out on various mailing lists for packages you use, you help out once and awhile. Have hints on performance tuning? Post a wiki page. Deploy some infrastructure you're proud of? Give a talk on it at a conference.
Many of these sorts of things don't fall under what most companies deem their "competitive advantage" -- and not to mention they get you contacts that can help you out of a bind when YOU need it, and are great recruiting tools...