Okay, I'll give it a shot, and if I get the concepts wrong, Tax or zddoodah is welcome to correct me.
mthelaw, if, in the course of their investigation, the plaintiff discovers PREVIOUSLY UNKNOWN information that neither side is aware of, and that information is favorable to the defendant, they are obligated to make the defendant aware of the information.
As phrased, that's not correct.
In general, a party to a civil lawsuit has no obligation to disclose
any information or document to the other side. There are two exceptions. First, a party may obtain discovery of facts, testimony and documents by serving written discovery requests or by taking depositions. Second, in some jurisdictions (although it appears Illinois is not such a jurisdiction) there are limited disclosure obligations that do not require service of a formal discovery request.
In the case of written discovery requests, the responding party is required to disclose responsive facts and documents known to or possessed by the party at the time the response is due. In the case of a deposition, the witness has no obligation but to answer the questions asked to the best of his/her knowledge at the time of the deposition. In either case, there is no obligation to volunteer information or documents that are not requested. Nor is there any obligation to disclose newly discovered information or documents after responses are served, unless the jurisdiction allows for discovery obligations to be continuing in nature. In Illinois,
Rule 214 of the Illinois Rules of Civil Procedure provides that "A party has a duty to seasonably supplement any prior response to the extent of documents, objects or tangible things which subsequently come into that party's possession or control or become known to that party." A similar rule applies to interrogatory responses, but not to deposition testimony.
Thus, in order for a party to have an obligation to disclose newly discovered facts or documents, another party must have requested such facts or documents via an interrogatory or document request.
However, the plaintiff has no obligation whatsoever to raise issues that are favorable to the defendant, if the defendant is aware of the information.
As I've written thrice previously, a plaintiff (or any party to a civil lawsuit) has no obligation to do anything at trial that is harmful to its own case. Once you get to trial, it's all about your own case. You emphasize the good and don't mention the bad (although a party that knows another party will offer adverse evidence may want to be proactive in dealing with it, but that's completely a strategy decision).