It is not uncommon for people to react differently when they are frustrated in public than they do in private. This dynamic of socially acceptable behavior is no different in an abusive relationship. An abuser might only mumble rude comments in public, but doesn't hold back in the privacy of their own home. When surrounded by people we know well we are all more likely to express our true feelings. This dynamic of using socially acceptable behavior does change the way an abuser will behave in public, so it is important to remember that they may behave differently behind closed doors.
Consider for a moment how sexual assault doesn't typically occur out in the open. People typically recognize this type of abuse as something that occurs with few to no witnesses. This perspective strangely shifts drastically when talking about emotional or physical abuse that don't leave any physical marks. For some reason it is assumed that these forms of abuse are witnessed, when in fact they rarely are. Strangulation is a common form of physical abuse that leaves very deep bruises that typically can't even be seen without an MRI. Marks from many physical altercations often won't appear on the skin for 24 hours and that is if they become visible at all. This is the main reason it is important to remember that if someone tells you that they are experiencing abuse you probably have not witnessed the worst of the arguments.
People involved in abusive relationships are wrapped up in a control cycle that is heavily weighted to one side. This is difficult to see because the victim will often make excuses for their abusers behavior. Often a victim will take the blame for the argument upon themselves to avoid further interrogation or suspicion from outside parties. This power control cycle can go on privately, slowing growing worse and worse for years. Abusers tend to have fairly normal relationships with most people, yet experience difficulty managing control or anger issues within their more intimate relationships.
It is okay to make the assumption that things are probably worse behind closed doors.
It is okay to ask if someone you care about is in danger.
It is even okay to walk away if they say that they are find and don't need (or want) your help.
However, it is never okay to assume that someone is not being abusive based on behavior that you personally witness.