We know that Death Eaters call Voldemort "The Dark Lord" (Harry confronting Snape in one the occlumency lessons, Order of the Phoenix).
In the seventh book, the Death Eaters jinx the name "Voldemort" to trace the ones who dare to use it, because the ones who use the name are people who are not afraid of Voldemort, like Harry or Dumbledore. I personally think they could do that after gaining power at the Ministry. The Ministry already has a tracing mechanism to catch people who use magic under 17 so they can modify it.* At which point did Death Eaters start being able to track people who say you-know-who's name?
So the Question is: after the mass breakout from Azkaban in Prisoner of Azkaban couldn't the Ministry of Magic use the same magic to trace Death Eaters like Bellatrix by jinxing the phrase "The Dark Lord"? I know that they didn't believe that Voldemort would be back, but still, Death Eaters escaped from Azkaban.
*Even though Voldemort himself did the Taboo magic, Dumbledore would knew the spell.
EDIT: I haven't read the books in their original language so I don't know when "The Dark Lord" or "Dark Lord" was used, please consider them as they have the same meaning (I know they don't) and consider the one with the narrower meaning(The one that is used to define specifically Voldemort).