The subject may be omitted because an appropriate identification is unavailable, or because the speaker cannot at present communicate exactly which person is meant. In that case the known action or characteristic may be conveyed without a definite name.
This differs from testing the listener. In the testing case, an indication is present and the listener is invited to use it. Here there may be a genuine limit on identification.
For example, a witness may be described only through the fact «شَاهَدَ الْحَادِثَةَ»—“[Someone] witnessed the incident”—when the speaker does not know the witness's name. The fact may still be useful, but the uncertainty should not be concealed. This is also distinct from reverentially withholding a known name.