gnus-summary-prepare-exit-hook

gnus-summary-prepare-exit-hook is a variable defined in `gnus.el'.
Its value is

(gnus-summary-expire-articles)

• This variable may be risky if used as a file-local variable.

Documentation:
*A hook called when preparing to exit from the summary buffer.
It calls `gnus-summary-expire-articles' by default.

You can customize this variable.