Are your visitors logged into Google?  Check for "sig2" parameter on the referrer string.

Useful if you want to customize messages/offers based on user account/email types.

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Parsing the Google referral string in a post (not provided) world – Q for Query
As of September 2013 Google prevented site owners from seeing all organic referring keyword data in the referral string. However there is still plenty of data to be gleaned from the string. For quick testing the HttpFox Firefox plugin is excellent. Systematically capturing the data is easily done in any web analytics tool or server […]

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This post was first made on the Richard Hearne Google+ profile.