A researcher collects data on household income and savings from 500 participants. After data collection, she notices that 180 participants (36%) did not report their savings amounts. Analysis shows: (1) participants who reported savings have an average income of $68,000; (2) participants who did not report savings have an average income of$67,500; (3) the missing data is scattered across all income levels with no clear pattern by income; (4) among high-income earners specifically, the non-response rate is identical to the overall rate (36%). However, qualitative interviews suggest that people uncomfortable with their low savings balances are reluctant to disclose savings amounts, even when they freely report income.

Which type of missing data mechanism is most likely operating here?