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Thanks everyone. I thought that was the case but it's always good to get a second opinion!

Have a read of this: Google: Your Content In Tabs & Click To Expand May Not Be Indexed Or Ranked

I'd have a look at the source code and see if the content is readable. Also copy and past snippets of the "hidden" text into Google to see if it is being indexed.

Thanks again for everyone adding their thoughts.

The traffic decline seems to have come about since Thursday 4th December. We did well out of the Penguin 3.0 update, having previously been negatively affected (link clean up and disavow put in place earlier this year). Our ranking on important terms have dropped below their pre-Penguin 3.0 uplift now though.

Our keywords have continued to drop again today with several showing a loss of 7-10 places (on top of previous drops).

I did test the expanding panels and found that Google did seem to be indexing the content okay. I have tried making one of the reviews panels permanently expanded to see if it makes a difference though but still worry it just makes the page look very spammy as the keyword is the same as the item being reviewed, so is repeated numerous times on the page.

Any further thoughts?

Thanks,
Kate