Who were the Arcani ?

[D B Campbell]

the missing books Ammianus Marcellinus

Not missing, Chance. The reference is 28.3.8.
AFAIK the manuscripts have "areani", but this has been universally emended to read "arcani" (= some kind of secret police). But some still believe that the manuscript reading should stand, and that these areani were some kind of irregular troops (like exploratores) quartered in the annexes (areae) of the outpost forts.

See, e.g.:
J.G.F. Hind, "Who betrayed Britain to the barbarians in AD 367?", in Northern History 19 (1983) pp. 1-7.
J.C. Mann, "Loca", in Archaeologia Aeliana (5th series) 20 (1992), pp. 53-55.