gnus INBOX housekeeping, kept (copies of) mail lying around

[Gijs Hillenius]

Doing some Gnus imap folders housekeeping this morning, I found that
8000 messages (called 131940.  131941. etcetera) /hiding/ in the imap's
INBOX. It looks like these are all emails that I've 'deleted' in Gnus,
so they don't show. But for some reason they're not actually deleted.

Is this a setting I messed up? I recall there used to be such a feature,
but I don't remember what that was called. I've been searching the
manual for this, so far without the result I hoped to find.

I would appreciate a few hints.