This is a deliberate reversal of Mahadevan's sign number 1018. The guard has his back turned, which would make him ineffectual. In context, it might be translated as relaxed, off-duty or off guard (in the sense of inattentive).
Illustrative Text Reference:
Mohenjo-daro: Seal: M-72 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 31: Collections in India: Suomalainen Tiedeakatemia.
Image Credit:
Indus Script Sign Number 1018 (mirrored): List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.