When the balcony collapses on Mouch, you can hear his Personal Alert Safety System (aka P.A.S.S. device) activating. But the sound you hear, the up and down whistle, is actually a warning alarm to let the firefighter know he/she has not moved in 60 seconds. If the firefighter does not move, the actual "firefighter down" alarm is 3 times as loud and is a 4 tone tweet.
The story that Olinsky reads to his daughter at the end of the episode is "Down The Rabbit Hole" from Alice in Wonderland.
The central event of this episode is based on the December 2016 Ghost Ship warehouse fire in Oakland, California. Although the fictional fire's fatalities eventually number 39, within the time frame of the episode, there are 36 confirmed deaths, the same number as in the real-life Oakland fire.
Part 1 of a 3 part arc, with part 2 being Chicago PD Emotional Proximity (2017) and part 3 as Chicago Justice Fake (2017).