A recent Professional Conduct Advisory Opinion from the Illinois State Bar Association (Opinion No. 18-01, January 2018) joined at least three other jurisdictions in concluding that the practice of using hidden email tracking software would be unethical for a variety of reasons. (See Alaska Bar Association Ethics Opinion No. 2016-01; New York State Bar Association Ethics Opinion 749; and Pennsylvania Bar Association Formal Opinion 2017-300.)
For more on this story go here, and here. Michael Kennedy, of Ethical Grounds, offers good analysis here.