Alexander Grothendieck's life and work is full of the wonders.
As far as I know Grothendieck largely decreased his mathematical activity after 1970 and became retired in 1988. However there are rumors about his private works and unpublished papers after 1970. Also there are strange rumors about some of his papers with pseudonymous names mainly between 1970-88 and even after it. For the first time I have heard about this in a free discussion in a model theory and algebraic geometry conference.
Question: Is it really true? Does Grothendieck have any pseudonymous paper? If yes, please introduce some references.