Hi Teodor,
Firstly let me say what a beautiful dagger you've got! Many congratualtions there on such a lucky find.
Bronze/Brass? Often very difficult to distinguish as both are copper alloys. From looking at your dagger, you have a mixture of copper alloys used in the hilt/scabbard. If you look at the end scabbard mount (Chape?) the lion (?) head looks rather gold and 'brassy' and seems to be cast then brazed to the chape which looks to have been made from sheets of a mostly copper metal which has been hammered and pierced.
The obvious differences made me wonder if (like mine) you dagger was once gilded or plated, but I cant see any traces even in the recesses?
The handle as Alex noted looks cast, possibly the fine detail added (chiseled in afterwards).
It would be unusual for bronze-rot/bronze disease to be active enough on a piece of this age to be leaving a hole, so I would think as Aex said that its much more likely a casting defect/blow hole.
If you look very closely at the 'damage' and can see a powdery verdigris coloured resudue over corrosion pitting then I guess it could be something sinister, but I'd doubt it.
Right! Backa gain.
I do like the choice of decoration, foliage and animals! Especially the rather monstrous lionhead!
Here is a rather nice heavy bronze tray of similar artistry:
