Current efforts to estimate the numbers of irregular migrants involve counting them at countries where they are believed to be in.
Let's think out-of-the-box for a moment, and invert that strategy.
Is it possible to count current irregular migrants based on data provided by their families back in their home countries? Why would families give away the location of their primary breadwinner? And how can we ensure that the data to be given by families is legitimate? Why would they provide false data? What can government planners offer as an incentive to families who will provide accurate, verifiable information?