The phrase "dropped out of sight" is commonly used to describe someone or something that disappeared suddenly and completely. However, there are several synonyms that can be used to convey a similar meaning. For instance, the phrase "vanished into thin air" suggests a sudden and mysterious disappearance without any trace. The term "disappeared without a trace" implies a sudden cessation of contact or communication with no explanation. "Fell off the radar" is another phrase that is used to describe someone or something that suddenly became untraceable or undetectable. Overall, these synonyms can be used interchangeably to convey the notion of a sudden disappearance without any apparent explanation.