"Catégories de Picard restreintes", Acta Mathematica Vietnamica 7 (1), p. 117-122, 1982, scans (pdf)
Grothendieck comments in a letter to R. Brown (5.5.82) :
"Also he [Quillen] had a promising approach to higher
K-invariants, which, he told me, was more or less
equivalent to a more computational transcription
of a somewhat abstract definition I had in mind
in terms of "enveloping n-Picard categories"
of a given additive category C, say, whose invariants
pi_i should yield the invariants K^i(C). (The
case n = 1 was worked out by a Vietnamese woman
student of mine around that time, Mme Sinh...)"
In "Récoltes et Semailles", Grothendieck writes :
"Un autre cas assez à part est celui de Mme Sinh,
que j'avais d'abord rencontrée à Hanoi en décembre
1967, à l'occasion d'un cours-séminaire d'un mois
que j'ai donné à l'université évacuée de Hanoi.
Je lui ai proposé l'année suivante son sujet de thèse.
Elle a travaillé dans les conditions particulièrement
difficiles des temps de guerre, son contact avec moi
se bornant à une correspondance épisodique.
Elle a pu venir en France en 1974/75 (à l'occasion
du congrès international de mathématiciens à
Vancouver), et passer alors sa thèse à Paris (devant
un jury présidé par Cartan, et comprenant de plus
Schwartz, Deny, Zisman et moi)."
Cf. also P. Deligne, "Le déterminant de la cohomologie", §4,
in: Current trends in arithmetical algebraic geometry
(Arcata, Calif., 1985), AMS Contemp. Math. 67, p. 93-177, 1987.
Grothendieck reports on his stay in Vietnam, Dec. 1967, scans (pdf)
Illusie relates the following.
"Once, Grothendieck told me, it must have been in '69:
«We have the K-groups defined by vector bundles,
but we could take vector bundles with a filtration of
length one, with quotient a vector bundle, vector bundles
with filtrations of length 2, length n, with associated
graded still vector bundles... Then you have operations
such as forgetting a step of the filtration, or taking
a quotient by one step. This way you get some simplicial
structure, which should deserve to be studied and
could yield interesting homotopy invariants.»"