Robbing is the most common crime on the street, where this particular app is applicable. It cannot be used in your home since you do not do much walking there and your phone may not be on you all the time.
When being robbed, the attackers rob your phone during almost all instances and then they switch it off or reset it immediately. Then, it won't be possible to assess the owner's orientation and velocity. I, therefore, suggest that the signaling device should be attached to something other than the user's phone, may be a pendant or something that fits in the belt buckle. Or a small chip to wear inside your footwear. Basically, the gyroscope and the signaling device should be in places that are usually not exposed during most crimes.
The phone could also have the app, which could be used during health emergencies, such as when the owner is hiking alone and falls or has a heart attack. However, the sensor should also be located elsewhere on the body.