Most of the beacons and bugs aren’t hidden at all. Email senders can tell from any image in an email (even a logo) if a message was opened and if so, from where.
Web beacons – for good or bad?
A web “beacon” or “bug” is simply something in an email that reports back to the sender that you’ve opened the mail. Such beacons are almost always images, and this is exactly why images are not shown by default by most email programs.
But perhaps most importantly, any image will do, and it certainly doesn’t have to be hidden.
For example, in my newsletter, I include the Ask Leo! site logo. That’s an image.
If you have images turned off in your email, you’ll actually see a line in its place that says “Please turn on images for the best experience.” That’s because there are often images elsewhere in the newsletter and you want to see those images along with what I wrote.
But that simple logo or any image, including my signature down at the bottom, can be used as a beacon. A “rich text” or HTML email that includes any image that needs to be fetched from a server on the internet in order to be displayed can use that image as a beacon – it’s the act of fetching it that tells the sender what they want to know. The only time you really need a secret or hidden beacon is if the email you’re sending didn’t have any images at all naturally. Then the beacon will be in the HTML of the message (view the message source or “original” to see that) and is often just an image reference like any other, though typically of a 1 pixel by 1 pixel image, or a transparent image.
If the message has more than one image, things get more complex. For some emails, there are going to be many images, and potentially from many different places.
Any one of them can be used as a web beacon. Heck, all of them could be. The point here is that you’re really kind of stuck. An email message can have images and displaying those images can be used for good (as I hope I do, to give you a better experience when you read my newsletter) or for not quite so good, because they can also be used to determine whether or not you’ve opened that email.
So unfortunately, I don’t think this really completely answers your question. Mostly because those bugs? They’re hiding in plain sight.