The Captain ( and the other two pilots) may well have seen that the THS was moving, as it does, usually and quite normally, untouched by human hand, but may not have noticed that the direction was (largely?) NU. A quick scan in the current circumstances could have failed to note the actual readings.
( My very ordinary car's oil pressure is indicated on a gauge, which I seldom watch. If the reading becomes abnormal I am alerted by a warning light so that I can take the appropriate action, pull over to a safe place.)
A modern aircraft like .......... should have something similar, when the THS goes to something out of its normal range, whatever that should be. (I might prefer a pulsing rather than a flashing light, adjacent to the THS.)