In this article from FutureLearn – Logical and Critical Thinking
The University of Auckland, this phenomenon is simply referred to as unconscious cueing.
Wilhelm Von Osten, a German high school mathematics instructor,
thought that people dramatically underestimated the intelligence and
reasoning skills of animals.
A man of science, he set out to test his hypothesis by teaching
mathematics to a horse called Hans. [the late C19] Hans quickly showed real aptitude;
soon he could read a number written on a board and tap his hoof with
the correct number of times.
... ... ...
Van Osten [went on to asking the horse questions (in German). He] would ask questions such as “If the first day of the month
is a Wednesday, what is the date of the following Monday?” Hans would
tap his foot six times. “What is the square root of sixteen?” Four
taps.
...
[This phenomenon was investigated by a young psychologist named Oskar
Pfungst.] Pfungst designed a careful set of experiments and began
testing Hans. He soon noticed that Hans performed well when questioned
under his normal conditions. But the horse’s accuracy dropped when the
questioner stood farther away than normal. And if the questioner
didn’t know the answer to a question or was concealed from him, Hans’
accuracy plummeted to nearly zero. Pfungst hypothesized that Hans’
cleverness relied on him having a close, unobstructed view of a person
who knew the correct answer.
With that in mind, Pfungst began watching the questioners, and he
noticed that as Hans tapped his hoof in response to a question, their
breathing, posture, and expression showed subtle signs of increasing
tension, tension which disappeared when Hans made the correct tap.
Innocently and without realizing they were doing so, Pfungst
concluded, the questioners were giving Hans a cue when to stop
tapping.
Pfungst had discovered ‘unconscious cueing’, the influence of
researchers’ subtle and unintentional signals on their subjects, and
it is now recognized as widespread in research involving human
subjects as well as animals.
I remember being warned about the need to avoid cueing answers when in teacher training.