The way we track opens is whether or not an image in the email loads. This can happen when an email is previewed or actually opened, and you can also get some false positives from some automated mail filtering software. Also, it is pretty common for folks to not load images and therefore NOT trigger opens even if they do them.
Sierra's comments are good for finding the when. My other advice is to use Click Link in email instead, it implies a high level of engagement and is more of a reliable event.