You'd be amazed how little dust accumulates in a place when people don't go there. That's because people are actually the source of most house dust.

Right. I've heard that a lot of it is actually dead skin.

That and lint from clothing and upholstery, pet dander, etc. All things that are associated with people living in the place.

Landlords in France do not have the right to enter an apartment they rent (with a few exceptions that were not applicable here).

Well two things come to mind.

1) Who is going to complain and know that they entered if the reason they are entering is that they can tell that nobody ever entered (say by placing something type of tell tale sign on the door showing no activity as is done in nursing homes?

2) They almost certainly have a right to enter for some exigent circumstances (that of course could be fabricated) fairly easily should they decide they want to enter the unit.

Well...

1) If the landlord has kept a set of keys, this possession would be illegal, and if he has not then he would have to break in the apartment, which would also be illegal. Sure they might have done it anyway, like any burglar could, but why?

2) In no case whatsoever the landlord has a right to enter without the permission of the renter or a court order (which could happen given reasonable suspicion that the place might be deteriorated).

Since the rent was paid, I fail to see why the landlord would have gone to such lengths to visit this place. He was most probably just happy to receive rent from someone who never had problems or complaints.

Interesting with respect to France (which I know nothing about so I will go with what you are saying).

But I am a landlord in the US and there is no such restriction and in fact it's written into the lease that I can enter. (Can this be written into the lease in France or it can't no matter what?) Actually by what you are saying "permission of the renter" it would seem that a lease could be written with this in advance, correct?

Anyway, it seems, from what you are saying, that the landlord in France is same as the police here. They can't enter without a court order but they can enter if, for example, a crime is being committed or if they suspect someone is in distress inside.

Separately (as far as France) I wonder what the distinction is between things that are illegal and things that are actually prosecuted. After all there are many things that are illegal in the US that are de minimis such that nothing will ever happen if it is "small potatoes".