Halliday & Matthiessen (2014: 252):
One of the interesting features of ‘macrophenomenal’ clauses is the form of what would seem to be the ‘receptive’ variant: instead of the expected the sand dredger heading for the cruiser was seen by him, where the whole Phenomenon is the Subject, we are much more likely to get the sand dredger was seen (by him) heading for the cruiser, where only the Subject of the non-finite clause serving as Phenomenon is ‘picked out’ to serve as the Subject of the ‘mental’ clause. For example:Halliday & Matthiessen (2014: 252n):
Smoke was seen billowing from the police headquarters after an explosion.This might suggest a different analysis of macrophenomenal clauses: what appears to be the ‘receptive’ variant could be interpreted not as a ‘receptive’ variant of a ‘macrophenomenal’ clause but rather as a clause with a verbal group complex serving as Process, was seen billowing, on the model of the sand dredger was rumoured (said) to be heading for the cruiser. Such constructions could be interpreted as markers of evidentiality – of the nature of the evidence for the information being negotiated.
A receptive variant such as the sand dredger heading for the cruiser wasn’t spotted by the navy would in fact be ambiguous: it could be a macrophenomenon, but alternatively heading for the cruiser might be an embedded relative clause. These two are significantly different in meaning. The interpretation as embedded relative clause would not be plausible where the non-finite clause occurs after the process of perception.