Where you see "innocent tracking code" some people see "exploiting a hole in the way a piece of technology works to obtain information about people that they don't expect you to be able to obtain, and never gave you permission to collect"
1.) Whether an email was opened
2.) When it was opened
3.) What IP was in use when it was opened
Is not public.
The fact that the sender of an email can exploit a loop hole in the technology so that they can gather that information, and the fact that it is commonly exploited to do so, does not make it ok.
I would agree with your Sherlock Holmes analogy, but only if being "naturally more observant" falls within the category of climbing through somebodys bedroom window, hiding under their bed and observing what they're doing without them knowing you're there.
