Personally, I would focus efforts in up to three areas.
1) Ensuring that the configuration / settings / versions / requirements etc that the system was validated under, and which are meant to ensure data integrity, are maintained. In small organizations this should be relatively straightforward. But the more interplay between dependent systems – for example integration with single sign on solutions – may result in dependencies where changes happen by someone not realizing that your validated system has particular needs.
2) Review problematic areas, ensure that resolutions have been implemented, or planned for. This can be precipitated by what are effectively “support” issues, in that if there is a high incidence of problems in a particular area caused by a user, it may be relevant to consider a technical solution that prevents those actions, or simplifies usage.
3) Security audit of accounts, to ensure that old accounts are disabled, or that changed positions haven’t led to more access than that position requires.