If a player disables his Shields within your sonar range either immediately or as a future event you will be able to see this graphically when the shields actually deactivate.
The time machine (in the future) only shows you things that you know. If an enemy has a plan to deactivate a shield in the future there is no way you can know this and therefore will not show up in your version of the future.
If they have deactivated a shield in the past then your time machine will show this because it knows what has happened in the past.
I think Braxo was saying that you can rewind the time wheel to see if the shields were dropped even though you don't get a notification to tell you somebody dropped the shields.