There are a number of synonyms for the phrase "unannounced departure," each with its own subtle nuance or shade of meaning. These include "sudden exit," which suggests an unexpected and swift departure; "abrupt leave-taking," which conveys a sense of haste and urgency; "unforeseen departure," which emphasizes the unexpected nature of the leave-taking; and "surprise departure," which implies that the departure was unexpected and may have caught others off guard. Other possible synonyms might include "unplanned departure," "unceremonious exit," or simply "disappearance." Each of these variations offers a slightly different take on the concept of a departure that occurs without warning or advance notice.