I do work on hospital managers panels at a secure psychiatric hospital. We often have decisions to make about detainability of patients who have been symptom-free for 6 months or more and have a good understanding of how to stay well in the community. The RC recommends that the section should be renewed so that a good discharge plan can be set up. This may have been taking 6 months or more already. I feel detention is not “warranted” in such cases. Should the lack of a discharge plan be grounds for renewing detention? Is there any helpful case law on this?
So firstly the test is whether it is mental disorder of a nature or degree that makes it appropriate for him to be detained for medical treatment.
The test is nature or degree so even with patients who are symptom free for lengthy periods of time the nature of the disorder may make it appropriate to be detained to receive further treatment such as psychology, drug and alcohol work etc or simply to plan a safe discharge and test it out via s17 leave.
I dont think it would be particularly difficult to argue that a section should be renewed so that a discharge plan can be made and so that the necessary testing via s17 leave can be done as well as and additional support that may be identified through a social care assessment.
In the tribunal context, the answer might be at least partly found in R (Ashworth) v MHRT; R (H) v Ashworth  EWCA Civ 923, in which the Court of Appeal said that when the availability of suitable after-care services is a prerequisite for the discharge criteria to be met, but the tribunal is in any doubt as to its availability, the tribunal should adjourn rather than defer discharge to a future date.
The adjournment part of this is relevant in your context: if you don’t believe the after-care under discussion is a prerequisite for discharge then you could just discharge straight away. Otherwise…
This is similar to an adjournment, which the Code of Practice mentions:
38.39 The presence or absence of adequate community care arrangements, including a deprivation of liberty authorisation or Court of Protection order may be critical in deciding whether continued detention (in particular) is necessary. If managers’ panels believe they have not been provided with sufficient information about arrangements that could be made were the patient discharged, they should consider adjourning and request further information.
38.40 If panels conclude that the patient ought to be discharged, but practical steps to put after-care in place (chapter 33), or obtain a deprivation of liberty authorisation or a Court of Protection order, need to be taken first, they may adjourn the panel for a brief period to enable that to happen before formally discharging the patient. Professionals should work together to minimise the time it takes to do this.
This tribunal case was considered in LW v Cornwall Partnership NHS Foundation Trust  UKUT 408 (AAC). Essentially, how soon the risk of relapse is likely to materialise is a relevant relevant consideration but not necessarily the only factor (the seriousness of the consequences being the obvious factor in the panel’s mind).