Train the sales reps to not load images, and not click links in Marketo emails that they sent themselves. It's an unfortunately side-effect of personalized tracking.
Also, I would not score on email opens, because it doesn't have any correlation with interest (most people load images because their email clients loads those images automatically, not because they actually read the email).
Jep's advice about training is the main preventitive measure. We constantly pound it into our reps heads (along with other best practices). We periodically have the reps clear their _mkt_trk cookie during sales meetings. We also have an alert email that is sent to me when a lead's IP address changes to the same as our internal corporate IP address, which can help me identify a specific rep that needs to delete their _mkt_trk cookie, but its not a 100% indication of who has the bogus cookie on their machine.
Thanks for the advice! I was trying to setup an alerts campaign like you suggested, but it doesn't seem to be working. Would you be able to provide some insight about how you set it up?
Right now my campaign has smart list:
Trigger --> Data Value Changes
New Value starts with (list of the main first digits of our corporate IPs)
Flow is send email alert.