Complex sentence: New documents concerning the arrival of Petzold and Pisendel in Italy have now surfaced but it seems very unlikely that Zelenka, not present on any of them, made the trip to Venice.
Split into simpler sentences: New documents concerning the arrival of Petzold and Pisendel in Italy have now surfaced. Since Zelenka was not travelling with his colleagues, it now seems very unlikely that he made the trip to Venice.