There are people with fear of unexpected strangers walking up behind them, frequently from PTSD. This can be crippling in outdoor walking situations. A bag/fann

There are people with fear of unexpected strangers walking up behind them, frequently from PTSD. This can be crippling in outdoor walking situations. A bag/fanny-pack sized device which can tap the wearer to let them know it really is time to check behind them would find a market… of some size… at some price.