It's possible you or one of your email recipients inadvertently copied the email tracking pixel (which isn't visible to the eye) and pasted it into another email.
An example is a user copied their email signature from an email where email tracking was enabled and accidentally copied the email tracking pixel when copying their signature. They then added their signature to Gmail's signature section, causing that unique tracking pixel to be sent out in all of their outgoing emails. It looked like the original email had been read hundreds of time.
The best way to fix is to re-create your Gmail signatures from scratch. We can also turn off email tracking on the original email thread if you'd like. Just contact us and we'll be happy to help.