For Insights, the Mobile Insights metric is based on the download of a specific tracking pixel image. This means even in preview modes, if images are downloaded to see the email, this can track as an open.
Two important things to note about Subscribers registering opens:
If the images do not download when a Subscriber is viewing their email, they will need to select the option to download the images in their inbox in order to register an open.
If Subscribers are viewing the email in it's preview pane, if they can see the images, they will almost certainly register an open.
If you have any questions at all, please don't hesitate to reach out to us at support@cerkl.com or use the support Chat toward the bottom right-hand corner of any cerkl.com page.