It could be that a property is being updated due to a change in how property values are being stored, so when you open the page something is updating even though you are not doing it explicitly.
Are you able to narrow this down to a particular document type or property? A quick way to test is to create a blank node of one of the affected types and see if the prompt still fires. If Saving/Publishing the node makes it go away then that may be your culprit but the difficulty maybe tracking down which property.
Justin