After further study, this is interpreted as probably the remains of a foundation cut for a wall of the eastern side of the Area A structure. The cut of a robbing trench for that same wall is also possible, although unlikely based on the composition of the fill.

Robbing trench south of wall SU420 continues for approx 1m before becoming uncertain. W edge is straight while E side slopes and is irregular. At S edge bottom of cut slopes from up from W to E side. Robbing trench - interpreted as such (but not securely).