I've Been Robbed!

A few things to keep in mind while tracking diplomatic units.

There is a built in wobble to their displayed movement on the global map.  You can see this in effect if you refresh  your map a number of times over and over as the unit will appear to jump about.

The closer to your city the unit you are tracking is the less accurate your trajectory/vector will be.  So it is important to follow and track for as long as your able to though after an hour or two you will have narrowed the options down to a very concise path.  This following and tracking is hardest for dwarf players due to the slower speeds of dwarven units but that is where your alliance/allies/friends/contacts come in.

The unit you want to send to track a diplomat unit leaving your city is an advanced spy unit because it has a 4 square visibility radius and is usually the fastest unit your race has with visibility.

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When following and tracking the further away from your city your can follow it and track it the more precise your trajectory/vector will be due to the above mentioned wobble affect.  Usually, it only takes a good hour or so of following to be able to pin point the likely sender at which point you can take a number of different actions to verify your suspicions.

The first area to go over is to ensure as exact a reading as you can.  My method is to ensure you have map grids on to help in your accuracy.  Just remember that the grid lines fall at the .5 distance of a map square.   The next thing is that you want to record the diplomatic unit at the smallest zoom level (0) so that only one map square appears on your square.  This will limit the affect the built in wobble has on your accuracy.  At full zoom(25) the wobble can jump the unit around by as much as 2 full squares.  At the smallest zoom the wobble will account for no more then +/- .1 in your reading.   To record the reading I will position my mouse following the moving diplomat and wait for the tick of the second hand to hit :00, :15, :30, :45 seconds.  I will then record the position of the mouse at that time estimating to the nearest tenth.  

I created a google spreadsheet to record these numbers so it will calculate the slope of travel and speed of the unit for me.   With this information, I can then enter in future times and predict where and when the unit will be on the map. You can then spread to your alliance members/allies/friends/contacts to help with tracking/following said diplomatic unit.   So, even if I were to lose track of the unit for a small amount of time it is easy enough to pick up again and follow given a few data points.  

You can also use the trajectory to eliminate any need to track closely because many times there are no cities for lengthy amount of times that the unit comes close to.  The other thing this allows you to do if you have access to the master city list is to find out any cities that have a similar slope to the unit you are tracking.  

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Putting this all together:

On your own, even as a dwarf, you should be able to get accurate readings up to an hour or even two hours out from your city.  If you are able to, take a readings every ~10 to 15 minutes.  I would allow at least 5 minutes between readings though.  With these calculations of slope and speed you can then plot a very concise trajectory/vector.  This will let you inform alliance members, allies, friends and contacts that are along the path that the unit you are tracking is travelling.  Up to and including the exact path and time they will be able to see the unit traverse within their city view.  I have done this on numerous occasions with great success.  You can also have these players send out clouds of advanced spies to extend their scope if needed.  Finally, if there is enough time you can attach advanced spies to army units(I suggest t1 cavalry commander and t1 cavalry) and camp them along the path of travel so that you don't have to micromanage a flock of spies.  Using this latter method you can also have a number of players camped out to watch said unit come home to roost.