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https://www.webmasterworld.com/google/4814310.htm [webmasterworld.com] by robert_charlton - 3:01 pm on Sep 1, 2016
(PDT -8)
I agree with you that features such keyword, location, browser type and so on (not set) may well be someone blocking the info for privacy reasons.But @ionguy my understanding is that whenever the hostname is set to anything other than your domain name then it must be a bot, this includes (not set). In so far as hostname specifically is concerned (not set) is always a bot and not a privacy issue. Filtering out the hostname (not set)s and other foreign domain from your data is essential to getting accurate stats.
This is applies to piwik and other analytics tools as much as it does to GA.