I believe this is passe compose with Avoir, so the past particle is not conjugated.
Hence Les cles ont disparu.
My French girlfriend and Google Translate concur on this.
In general I love this course. It has a few wobbly corners and some shockingly incorrect audio. But it covers a wide range of French, mostly in a sensible context. I find this much more useful than, say, learning verb infinitives with dodgy or confusing English translations. I think the approved Memrise course creators could learn from this course.
I also think that explanations of the grammar involved in each sentence would be far more useful than memes. This course covers a lot of grammar in a subtle way that may be lost on a lot of people.
I would be very happy to contribute to improving this course, if it is possible in some way. I could probably arrange some real French audio, for example.
I tried to tag the course creator but he is not registered on this forum. Maybe @MemriseMatty can tell us if he is active or if he has administrator on his course
You are right about the conjugation, in any case.
Les clés ont disparu (conjugated verb)
or
Les clé disparues (adj.)
About the explanatory mems, I suggest that you create them for yourself and, by extension, for others
I believe you are right with the correction because it is not necessary to have gender or plural agreements with conjugations in avoir, though, you would add agreements for être for passé composé.
Is there anyone contributing to this course? Nazar doesn’t have a forum account so there’s no way to contact him directly. As much as I like the course’s content - the audio is just so wrong, it would be better to get rid of it than to have it!