This, this, a thousand times this.
You can look at the source code all you like, but unless you can *use* that source code to build your own binaries and redistribute them, then that means absolutely nothing in terms of security.
The products you buy off the shelf may or may not have any relation to the code you looked at.
That's why Free Software is so important for security-sensitive applications. Not only do you get to look, you get to modify it and redistribute.