When someone says they are a witness to something, it means they have seen or heard something firsthand. There are various synonyms for the phrase "are witness" that can be used depending on the context. Some of these include "observed," "viewed," "witnessed," "noticed," "perceived," "experienced," "encountered," "detected," "spotted," and "sighted." These synonyms can be used interchangeably to convey the same meaning. Using synonyms can also add variety and depth to your writing or speech, preventing repetition and making your language more engaging.