Yes - I always understand that diplomatic license plates do not show the name of the owner's country. So that the vehicle won't be a target of someone in the US who has a grudge against that county. So how can you tell they were from Guatemala?
And yes, foreign diplomats in Washington are by law confined to always staying within a certain number of miles radius of Washington.
The tag numbers contain a code that indicates what country they are from, but it's not known to the general public. That doesn't explain how (or if) the OP knows the code.
I've never heard of foreign diplomats being confined to the area around DC. I have trouble believing that for a number of reasons. One is a catch-22...if you are saying they would be breaking the law to leave a certain area, then couldn't they just cite diplomatic immunity to such a law? Perhaps you are saying that immunity doesn't count outside a certain radius. If that's the case (which I also doubt) then maybe they decided that they would go for a drive and not break any laws. Another reason I doubt there is such a restriction is that it could severely limit the diplomat's ability to do their job. For example, many diplomats are charged with protecting the well being of the citizens of their home country, while that citizen is visiting a foreign country. If those citizens are outside of a given radius, are they then outside of the diplomat's protection? Maybe in some countries, but not in the US.
Also, there are more diplomats in New York than in Washington. If there is a radius around Washington, then there must be one around New York. If these radiuses overlap, they negate their purpose. If they don't overlap, then what about diplomats that travel between the two cities? What about consulates in other cities?
Also keep in mind that children of diplomats also have immunity in most cases. One summer, I worked with the son of a French diplomat. He was a student at University of Maryland. Also, at my first apartment, just north of Baltimore someone who lived in the complex had a car with diplomatic tags. It was a beat up rust bucket, so I doubt it belonged to an actual diplomat, but more likely to the child of one.