Unmasking bias occurs when an exposure causes symptoms, resulting in the search for a disease.
For example, a large number of patients might develop severe GERD after eating spicy burritos. Because of their GERD, they are more likely to undergo endoscopy than the general pubic. During endoscopy, a number of patients are found to have gastric cancer, and statistical correlation is identified, despite there being no real association between burrito intake and gastric cancer.
Unmasking bias is also referred to as detection signal bias. It is a type of selection bias.
Sackett DL. Bias in analytic research. Journal of chronic diseases. 1979; 32(1-2):51-63. PMID: 447779