BBM
I'm not sure if it was ever stated that his last day was August 31st. After the shooting, the Tribune reported that Fox Lake Mayor Donn Scmidt, a close friend of Lt G, said "He'd recently discussed retiring as soon as this month, said Fox Lake Mayor Donny Schmit."
http://www.chicagotribune.com/suburbs/lake-county-news-sun/news/ct-fox-lake-school-lockdown-20150901-story.html
So we know that in Arpril 2015 he reached 30 years of service and the corresponding 75% maximum pension. He has also been applying for several police chief positions. There was probably a lot of informal discussion and speculation regarding his retirement which is typically for employees with 30+ years. Lt G probably had not made a concrete decision on a retirement date because he would like to have another job lined up before he formally submitted his retirement papers.
The selection process the position of police chief involves several rounds of interviews and background/reference checks and candidates do not know when a final decision will be announced. So I believe that Lt G's statement that he may retire as early as September was related to his hope that he would get the Antioch job. That did not happen and Antioch swore in their new Chief on August 17. Shortly thereafter, Fox Lake Chief Behan was suspended and days after that he chose to retire. Days later, the mayor meets with Lt G and reportedly asks him to continue working until the end of September while they seek a replacement for the Chief. Because he did not state the following, I am assuming that the Mayor did not ask Lt G to serve as interim chief or encourage him to stay and apply for the position of chief. This despite their professional relationship and close friendship of 30 years.
It is likely that Mel G knew that her husband was not being considered for the position of chief by Fox Lake and was understandably unhappy. I think her comment reflected that, with the benefit of hindsight, she wished that he had just walked away from Fox Lake and collected his 75% pension while continuing to search for a police chief position.
I have no real theory regarding the text message that the mayor reportedly sent. That doesn't make sense to me on several levels.