In case it was a S-360 check airliners.net for "Nightexpress" - although I have never seen it at STR.
You mentioned "high wing" and "no struts" (although you were not sure on this one). What about the AN-26 opb CityLine (cargo operator from Hungary)? They visit STR quite often.
I'll visit STR next week. Maybe I'll see what you saw
