The Return to Valbona

To find the answer, one must walk along the edge of an unfathomable abyss.
Three students from the Czech Republic go on a mountain hike in the Prokletije range for their vacation. And they disappear. The families are inconsolable. The government,... radio, and television join the search. But not even Interpol can find a trace of the young people.
Several years later, Jana receives an envelope containing the passport of her missing sister. Among the postal stamps is one that cannot be placed on any map — Valbona 666. Could it be that her sister is alive and sending a signal? Is it still possible to save her? Otherwise, who and why would need to send her passport?