The picture of the waves in the upper bedroom changes during the movie. The tide is rising. Same applies for the black and white picture in the living room which resembles a changing Rorschach' picture.
Kevin Bacon's character is a very detailed prepper. He has a willow tree growing in his yard. Acetylsalicylic acid (ASA / aspirin) is synthesized from willow bark.
Amanda's laptop and the emergency kits that Danny buys at the store are made by E-Corp, a fictional corporation that was central to the plot of the previous project by writer/director Sam Esmail, the series Mr. Robot (2015). A book "Beach Towel" written by another Mr. Robot character, Irving, also appears.
GH's neighbour's last name is Huxley, an homage to Aldous Huxley who wrote the 1932 Brave New World novel, amongst others, that painted pictures of dystopian futures where technology has become the thing that corrupts humanity and is used as a way to harm people.
The wall art in the master bedroom changes each time the mood intensifies, according to the artist Adam Hall.