They should be looking for a "military road" running the length of this "line" just behind the line of forts…
This "system" was nothing new. Following the early invasion/occupation of South-east Britain, a line of forts (and fortlets and signal towers too?) stretched along a line that ran "in front of" (northwest of) Dere Street, "anchored" in the southwest on the legionary fortress of Exeter and in the northeast on the legionary fortress of Lincoln.
Hence I say, where is a "military road" running the length of this "line" alleged in Scotland????? 
The abandonment of this line in Scotland seems consistent with all we know (or think we do) about the period subsequent to the governorship of Agricola. Pressure from the Dacians on the Danube (was this when Legio V Alaudae was wiped out and removed from the army rolls?) saw a shift of army units from the Rhine to the Danube, and it seems from Britain too. The army in Britannia withdrew from Scotland. And built…?
…Built the Stanegate, which initially served as a strategic road between east and west through rough country, at least linking two important river crossings and forts; Corstopitum or Coria (Corbridge) on Dere Street in the east, and Luguvalium (Carlisle) in the west. Following the full withdrawal from Scotland it was improved and more forts added and it became the new frontier -- with forts at intervals of a half-day's march. There are a few "fortlets" known, IIRC, but even today more work needs to be done on this strategic line; work made difficult by the subsequent obscuring of the terrain by the building of Hadrian's Wall…
Trajan's Via Traiana Nova was a line of communication running east from Heliopolis, on the Nile, through the Mitla Pass in Sinai, to Eilat and Aqaba, thence north past Petra, Philadelphia (modern Amman), Bosra (the legionary base), and Damascus, to terminate on the upper Euphrates at Resafa. A number of studies on Rome's eastern desert frontier have commented on the role of the Via Traiana Nova as a line of strategic communications behind a line of fortresses and forts much like Dere Street, "Britain's Third Wall (sic)," and the Stanegate.
"Wall" is clearly a mis-nomer for this "discovery." However, this sounds nothing at all out of the ordinary. It also sets the legionary base at Inchtuthill in an even better context, as the legionary "anchor" of the line. Wish I could visit!!! 
