Man, that's such a shame, especially since you already know the police that you'd have to turn it over to (especially that one guy that keeps poking around) would either take it for themselves or sell it...
Moral of the story is: gun laws in Europe are a PITA. You can be sure criminals who buy and sell on the black market don't care about registration!
We used have pretty loose gun laws back before 2006. Little restrictions or oversight. In 2006 an individual went on a shooting spree in Antwerp with a shotgun and the government sped up gun law changes that were already in the works and also added more restrictions. It was a very drastic change in restrictions on what kind of firearms you can buy, why you can buy one, changes to the firearm permit system, reclassifying certain gun types, ...
I was only 12 at the time but I remember that shooting spree sent a real shockwave through the country. As the law was enacted, many gun owners were no longer eligible to own certain firearms or had to renew their permits under the new system. The government and police also urged citizens to voluntarily hand in their firearms, which a surprisingly huge amount actually did. The police would pay for firearms you were no longer eligible for under the new laws, unregistered guns, illegal guns, ...
without penalty quite some time. An electronic firearm database was established and vastly expanded to make monitoring ownership, sales, permit expirations etc. much easier.
There wasn't really much of a "we need these guns to defend ourselves" mentality as I recall it, the main focus was hunting and sport shooting. Hence the large amounts of guns that were voluntarily turned in. Of course many guns from that pre-2006 period still remain because the gun laws were so loose compared to post-2006. I live in a small rural town with numerous hunters around here and I can certainly think of a lot of people who still have unregistered hunting guns from that period. I know my dad only turned it one or two he no longer wanted anyway, and the police paid him for them. But he always had at least 1 or 2 unregistered guns that I knew of. If you made an authorized gun purchase after that 2006 law, the gun is bound to be in at least one database. That wasn't the case before 2006, so it's hard to say how many unregistered guns from that period remain in rotation or even registered guns that were simply not added to databases at that time.
I think our restrictions and electronic monitoring of all gun purchases aren't a bad thing but they should make things easier for people without firearm permits like me who inherit a bunch of guns. It's not the beneficiary's fault if you inherit a bunch of unregistered guns or registered guns without proper paperwork. The process of simply trying to find out if there's any record of them being registered at one point is already tricky and can result in search warrants for your house if not handled carefully. I can't do so myself, search up serial numbers or anything so a third party needs to do so. Which would be either an authorized gun store or someone whose job involves having routine access to said database and doesn't ask too many questions. In other words, probably someone in law enforcement or government.
I think a better option would simply be to let people like me formally register any inherited guns no matter their status, thereby bringing any unregistered ones them back into the system for resale. One positive under the current law for people in my position is that we have a several months period where you can't really be charged for possessing illegal/unregistered firearms that were inherited within that period after the previous owner's death. The only way you could be charged with something in that case is if police catches you in the act of knowingly possessing unregistered guns and deliberately concealing them. The police are definitely known to come knocking early for information on inherited guns when someone in the firearm database dies but going to the lengths of search warrants is pretty rare from what I hear from the gun store. And even then, the police usually just confiscates the guns and gives you a formal warning.
If you get caught after that 4 month period though, then you'll likely be looking at real charges.
The loophole everyone uses if they can't find any paperwork etc. for an unregistered gun and are forced to turn them over to the police at the end of the 4 month period is to take them to the police and simply claim that "you found them after a more thorough search". Nothing the police can do about that, they simply have to take your word.
So that's the plan. My dad died on June 26th so I have some time left to find some form of paperwork or proof of registration. If that fails, they'll be turned over the police. One individual has already offered to "buy all my guns" but that's practically begging for legal trouble.
There is also a third option that does allow you to registered unregistered firearms you inherited (provided they would be legal if registered, so not that shotgun with the fully backwards folding barrel) but it is so complicated and time-consuming that you might as well just hand them over the the police. Through a notary, you can contact a special provincial committee to register your guns. But since you not only need to pay a notary to handle the process of the request, communicating with the provincial committee, ... you'll probably have spent more money on notary fees than any price you'd get being able to resell the formerly unregistered guns. This option is mostly for people who already have a firearm permit and want to be able to legalize guns they inherited.