This is the outline of an area to which the dot of emptiness has been added. The dot of emptiness signifies that something is not known. Hence, the meaning of this Indus sign is unfamiliar or new
Illustrative Text Reference:
Mohenjo-daro: Seal: M-139 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 45: Collections in India: Suomalainen Tiedeakatemia.
In inscription M-139, Mahadevan's sign number 2185 is full character height.
Indus Script Sign Number 2185: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.