The thing is that this lichen was needed on Earth as a reliable and tested medicine to prolong the lives of honored and notable people threefold. However, this method of extracting and delivering of the lichen to Earth was extremely expensive and took a long time, plus its existence on Abba was very scarce. And sometimes on the transit back home, the miners of that valuable cargo would eat some of it themselves to prolong their own lives, despite a strict law forbidding mere mortals from doing so under penalty of death. The first offenders had been dealt with precisely that way – upon returning to Earth they had been summarily executed by firing squad to set an example for others.
And these prohibition violators were caught quite easily: those who consumed several grams of that valuable lichen would develop a persistent and lingering garlicky breath, like all governing and elite citizens of Earth who partook in the ambrosia made from the lichen. The odor could be temporarily masked with strong aromatic substances or by eating plenty of fatty foods, but as soon as the unpleasant smell was no longer covered up by these methods, the garlicky breath would come back to the super-centenarians. And since this persistent smell was practically impossible to get rid of, people began to identify the elite super-centenarians by their distinctive stench. And not just that – by mandated laws for the people, the stink gradually began to be viewed as an aura of pomp and wealth. Naturally, regular Earth folk were not supposed to smell this way, so a whole series of regulations and decrees were passed prohibiting people from growing and eating garlic and its byproducts, so that their breath would not be confused of that of the governing elite.
As a result, workers and crews who had come back from the off-world lichen mining missions were easily probed during quarantine; anyone who was detected with the garlic breath was detained and executed. However, it was noticed later that within the prison cells where the ambrosia eaters had been kept, various insects (cockroaches, caterpillars, fleas) and other bloodsuckers would thrive for a long time even though the spaces were empty and had been disinfected multiple times after the criminals had been executed.