"Part of Hammersmith Bridge was closed off for seating to watch a regatta. Mr Bird insisted on walking on that part of the Bridge and climbed into the enclosure. He was prevented from getting out at the other end. There was nothing to stop him from going back the way he had come and crossing the Bridge on the other side which had not been closed off."
In this case it was said that, "[a]prison may have its boundary large or narrow, visible and tangible, or, though real, still in the conception only; it may itself be moveable or fixed: but a boundary it must have; and that boundary the party imprisoned must be prevented from passing; he must be prevented from leaving that place, within the ambit of which the party imprisoning would confine him,"
This ruling contains some obvious points and a not so obvious one. The prison may be "in conception only". That is, it may be entirely imaginary, such as being placed under arrest by someone who hasn't got the power to do so.
What about if you have been imprisoned but you don't realise it?
In the following story,
"Women rescued from fake reality show.
ISTANBUL, Turkey — Turkish military police said Thursday that they had stormed an Istanbul villa to rescue nine women held captive after being tricked into believing they were reality show contestants."
... were the women falsely imprisoned?
Imagine that the charade had reached its conclusion and the women were allowed to leave, would they have been falsely imprisoned? Assuming that they were being imprisoned throughout the charade, then, even though the women were not aware of their imprisonment, they were falsely imprisoned.
It was found that "a person can be imprisoned while he is asleep, while he is in a state of drunkenness, while he is unconscious and while he is a lunatic" by Atkin LJ in Meering v. Graham White Aviation Co.Ltd.  122 Law Times 44.
Further, it was said that, "I cannot agree with the Court of Appeal that it is an essential element of the tort of false imprisonment that the victim should be aware of the fact of denial of liberty" in Murray v Ministry of Defence (Northern Ireland)  UKHL 13.