Good Afternoon Ali,
The Protocol at the back of the Standard has the relevant clause - Part 3 Section 1.6.5
The key aspect is whether you are managing/owning the storage operation. If you are, your next BRC audit will include an inspection of the warehouse unless it is more than 50km away.
If it is subcontracted, you treat it as a supplier as in clause 5.10.5 of the Standard (High) or 5.10.4 (Basic)
Although it will be mentioned in the audit report, it is probably not necessary to include it within the wording of the scope.
I would certainly put it in your HACCP and if it is adjacent monthly inspections would be good.
The warehouse may not be up to the same Standard as your own factory so make sure you can risk assess and justify any difficiencies.
If you are managing it, have you extended your pest control contract?