Topic drop occurs when a speaker leaves out an element that has moved to the "sentence-initial" position, typically because it was already mentioned in the previous discourse.

In linguistics, is a phenomenon where the "topic" of a sentence—the person or thing being talked about—is omitted when it is easily understood from context. This often results in sentences that begin directly with a verb (V1 structure). Core Mechanics of Topic Drop

The frequency and rules for dropping topics vary significantly between language types: Topic drop in German: Grammar and usage - ResearchGate

It primarily happens at the "left edge" or beginning of a clause.

Topic drop in German: Empirical support for an information-theoretic account to a long-known omission phenomenon

The omitted information must be predictable and easy to recover for the listener.