You can create a global variable, specifically an array of strings, into which you insert the objects which are in the selection list. So, when a user selects a new object, you compare the new selection list with the list of objects which are stored in the global variable, and the ones that are missing in the new selection list have been removed, which is what you wanted to know. Then you store that new selection list into the global variable to prepare it for the next comparison which should happen when the selection changes again.
I believe that you can keep track of selection list changes with the help of a script job, which could trigger a user defined procedure in which you can do the stuff mentioned above. The documentation should provide enough information on creating an adequate script job.