The term is a hapax legomenon, known only from a passage in Ammianus, and it has been suggested that it is a misreading of "arcani", the secret ones. These may have been civilian inhabitants of the paramilitary zone between Hadrian's Wall and the Vallum to the south.
There is also a Vindolanda tablet 162 that seems to provide support for this theory. These tablets were excavated from the Roman fort at Vindolanda in northern England.
This Tablet 162 is a fragment of a leaf containing two words written in a good capital hand. It seems to read: MILES ARCANUS, so it might refer to an intelligence agent or explorator of some sort.
Ammianus text goes as follows.
Ammianus Marcellinus Liber XXVIII III VIII; A.D. 368 28, 3, 8.
...haec etiam praecipua. Arcanos genus hominum a veteribus institutum, super quibus aliqua in actibus Constantis rettulimus, paulatim prolapsos in vitia a stationibus suis removit: aperte convictos, acceptarum promissarumque magnitudine praedarum allectos, quae apud nos agebantur, aliquotiens barbaris prodidisse. id enim illis erat officium, ut ultro citroque [per longa spatia] discurrentes, vicinarum gentium strepitus nostris ducibus intimarent ...
"During these outstanding events the areani, who had gradually become corrupt, were removed by him [Theodosius] from their positions. This was an organization founded in early times, of which I have already said something in the history of Constans. It was clearly proved against them that they had been bribed with quantities of plunder, or promises of it, to reveal to the enemy from time to time what was happening on our side. Their official duty was to range backwards and forwards over long distances with information for our generals about disturbances among neighbouring nations."