I suggest checking Tracking settings in your Sendgrid account. I found myself in a similar situation with a tracking pixel being embedded in my personal email notifications. After checking the settings page, turns out Open tracking and Click tracking were enabled, even though I always disable everything spyware-related in every account I create. I can only assume they were silently added and enabled at some point during the last couple of years.

As you can see from the blog post, those tracking settings are/were disabled. As a developer I know that it's easy to leave wrong default values on checkboxes so I went to first disable them (it was enabled by default) -> enable -> disable.

The tracking pixel was added in test emails as well as in the actual emails nevertheless.

I'm not arguing with the premise of the blog post. In my case, the emails were send as "single-send", not a part of a campaign, so disabling the options worked. But the blog post was very helpful nonetheless, as it lead me to discover the mysteriously enabled tracking options, which I would have never got around to checking myself.

Even better, I checked the actual HTML received from a few recent campaigns - no Sendgrid tracking either at link or image level, luckily! :-)