I read through a number articles and lists of problems people with disabilities face, and was intrigued by one I found that mentions that deaf people are often jumpy because they are regularly surprised by people coming up behind them. https://www.ranker.com/list/things-deaf-people-have-to-deal-with/nathan-gibson
If deaf people could be discreetly be alerted of people approaching them from behind they would be startled less often.
Proof of Concept
An Arduino (in this case a Sparkfun RedBoard Edge) connected to a human sensor to detect humans, a motion sensor to know when the person wearing the device is moving (to ignore humans while in motion to prevent false positives), and a vibration motor to silently alert the wearer of the device that someone is coming. By putting this all in a small case that clips to the back of someone’s belt, it should provide some warning of approaching humans.