Border control, like many areas of public/civil security suffers from a common problem: a lack of reliable information allowing judgments of how effective it is - we only know what we detect. The relationship between 'real' illegal flows and detected illegal flows is a well-known "known unknown" and yet, there are several possible ways of gaining better information in regard to this problem. For example, better guesses on the size of real illegal migration flows could be gained in several ways, including red-teaming, introduction of random patrols on top on scheduled ones, using mathematical models, or interviewing 'successful' illegal immigrants on modi operandi to list just a few.