OTOH, the rumors are most often of the "my 2nd cousin's mother-in-law's half-brother's secretary's boyfriend's ex-husband" variety, ie. not even anonymous 1st hand claims.
Anecdotally, these rumors seem to most often be spread by Enterprise SaaS salespeople.
OTOH, "The NSA has a backdoor into Google" used to be something only paranoids believed too...
You can't just decide "Hey, I've got an idea for a cool 20% project, I'm going to try crawling all mail and looking for stock market tips". There is defense in depth, in terms of technical walls, as well as people walls, that makes something like that very unlikely. With tens of thousands of employees, it would be easy to get 1 rogue employee who could do serious damage with that kind of access.
Google takes data security and privacy very very seriously, and a breach of that magnitude would threaten hundreds of billions in valuation for a rather crazy risky idea for an M&A department?
By looking at data from Crunchbase, Angelist, etc you can get a deeper graph of the interrelationships in Silicon Valley. If I was looking at who to invest in, or M&A, I'd be looking at data like that, as well as talking with investors and people they've worked with.