A stock may be put under surveillance for non-promoter encumbrance when entities or individuals who are not promoters have pledged a significant portion of their shares as collateral for loans or debts, which can create potential risks for the company and its shareholders. When a stock is under surveillance for non-promoter encumbrance, the stock exchange or regulator is monitoring the situation closely and may take action if necessary. Investors should carefully consider the potential risks and benefits of investing in such a stock and monitor any developments related to the encumbrance situation.