An investigation is under way into why an elderly patient was locked in a bedroom at hospital psychiatric ward.
Laurel Ward at Bristol's Callington Road Hospital was shut following the incident in June, and 15 patients were transferred to other hospitals.
Avon and Wiltshire Mental Health Partnership NHS Trust papers reveal the patient was "unlawfully restricted".
The trust has refused to comment further pending the conclusion of the investigation.
It added the ward will remain closed until "a later date".
In a statement, it said: "Laurel [Ward] was closed due to an issue of concern, when a patient was locked in their room for a short space of time. Although they were not at risk of harm, the action was not in keeping with our values and culture.
"Due to the nature of the incident, we were required to have the police investigate, which they did and found that no offence had taken place."
Avon and Somerset Police said it could find no record of having carried out an investigation.
However, the board papers show officers investigated the allegation during July.
The report also revealed a training programme has been put in place for Laurel Ward staff and at the hospital's Aspen Ward due to "similar concerns" being raised there.
Following the incident in June, the Care Quality Commission (CQC) said: "We are satisfied that the trust has taken steps to ensure the welfare of the patients."
It has yet to publish a report following its most recent inspection of the unit on 26 June.
In 2014, it made an unannounced inspection following concerns from Avon and Somerset Police about patients absconding.
While inspectors found most standards were being met, it said poor record-keeping enabled several patients to leave the psychiatric unit without permission and made recommendations.
The 122-bed hospital opened for psychiatric in-patients in 2006 replacing Barrow Hospital in Somerset.
Mental health services run by Avon and Wiltshire Mental Health Partnership NHS Trust were put out to tender in 2014 after complaints about poor service.
Mental Health Bristol, a partnership of 10 organisations, took over the majority of services.