You should always ask before going through someones phone.
Tammy and her friend are trying to decide where to go out to dinner, and her friend leaves briefly to use the bathroom and leaves her phone behind.
Tammy wants to choose a good restaurant.
Tammy waits until her friend returns from the bathroom and asks if they can use her phone to look up restaurants on Yelp.
Tammy grabs her friends phone to look up restaurants on Yelp, and accidentally sees sensitive pictures her friend has sent to her boyfriend.
Tammy's friend is happy to look at reviews and choose a restaurant together.
Tammy's friend is embarrassed and angry that she used her phone without asking.