Dear Customer,
to be able to log in to a company a user must have a Person record in that company or a Global User record with a reference to that company. If the user only has a Person record in company A and no Global User record (in any company), and can still log in to company B, then this could be a bug.
Please, send us a version number (and date) you are using so we could test this scenario.
The system should not be affected from a technical point of view, yet your data may be available to users who should not have access to them. I wouldn't expect any error message.
You may check the system log (hansa.log file on the server) to see if there are any specific messages at the moment the user enters the company he/she should have no access to.
HansaWorld Support Team