A Montenegro Airlines Fokker 100 is thought to have been hit by a stray bullet while on approach to Podgorica at the end of a revenue service from Ljublanja, Slovenia on 6 January.
In Montenegro the Epiphany, which occurs on 6 January, is sometimes celebrated by shooting rounds of ammunition into the air.
Although the bullet struck a tail surface the aircraft landed safely. The damage was discovered during a routine post-flight check.
Police investigators examined a "3cm [1.2in] hole in one of the service openings in the tail area" and said it had been "very likely done by bullet round". The aircraft was grounded for one day for repairs.