Halliday & Matthiessen (2014: 303, 304):
‘Saying’ has to be interpreted in a rather broad sense; it covers any kind of symbolic exchange of meaning, like the notice tells you to keep quiet, or my watch says it’s half past ten. The grammatical function of … the notice, my watch is that of Sayer.
… unlike ‘mental’ clauses, ‘verbal’ ones do not require a conscious participant. The Sayer can be anything that puts out a signal … In view of the nature of the ‘Sayer’, verbal processes might more appropriately be called ‘symbolic’ processes …