In Northumberland's Prehistoric Rock Carvings (Pendulum, 1983), Beckensall says 'The site is very unusual at the side of a stream, and does not command the usual extensive views', this is closely tied in with his Zen-like statement in Prehistoric Rock Art in Northumberland (Tempus, 2001), when he adds as a caption on the photograph on p.119, "This flat boulder is either in it's natural position, or has been brought there". I'd agree that it seems to be in an incongruous setting, but noticed that when approached from the east, you can see above the treeline and there's a superb view over to Cheviot, framed by the two nearest hills. This argues to it being in situ.
However, when you're at the boulder itself, there's a profusion of field clearance stones, all of which are either small cobbles, or medium sized bits about the right size for a Northumbrian kerb cairn. So there's the possibility that the marked stone came from a now defunct cairn. The area drips with funerary remains.
Adding to the ambiguity is the fact that the stones seems to lie smack bang on a prehistoric trackway, complete with standing stones, cists etc., which supports either viewpoint. I just couldn't make my mind up completely.