This may not be specific to Prestashop but it is happening on a Presta site so I'm posting it here

Last week Google webmaster tools was showing 64 indexed pages for a site now it has gone down to 57.
I haven't removed any pages or products, I have always had the robots file set so that checkout, account pages are not crawled and I have had the SEO urls mod on since day one.
The only thing I can think of is that I added 10 products to the site about 3 weeks ago then had to deactivate them within a few hours of them being on the site. So technically they are still there but the category they were in and the products themselves are not active hence cant be viewed or accessed. Is it possible that Google indexed them within a couple of hours but has now taken about 3 weeks to realise they are no longer there and has started removing them?