The old wacraft alpha tracking ability allowed to you to see where enemy units had been. As if the nifty footprints units leave behind stayed and showed on the minimap, basically. Thus, you could figure out where the enemy had been, but you didn't have a complete up-to-date thing going on, like you would with any of the abilities found in W3. But, they dropped it...
Anyways, I figured out how to do it with triggers. However, this requires that you sacrifice the funcionality of one of the two invisibility types (burrowed or normal).
Basically, you have a trigger where every n seconds it picks all units and creates a special unit at the first unit's location. This special unit would have the ghost ability, perhaps look like footprints or anything else you'd like, and would also have the "locust" ability so that it would be unselectable. Then, you add an expiration timer to the created unit (It's an action in the Unit section), setting it at x seconds. Finally, you make an ability for the tracking unit, which allows it to see burrowed units whenever you want it to. Viola!
That's the basics. You could go deeper, and try to show the age of the tracks to the player by how much time is on the expiration timer, etc. I'm SO glad I figured it out. | 
