In order to persist per-user data and settings, the web service will create and never recycle per-user folders under each model folder within the offline folder as explained in 8.2.2 Model files on the server. While this is manageable for a small user group with well-defined identities, it is certainly a problem if the web service reverts to using temporary session ids as these folders will just persist indefinitely!
Automatic housekeeping for such temporary session data may be added at a future date, but for now we strongly recommend a monthly task for the site administrator to review the disk usage under My-eSTEM and be ready to do some disk housekeeping if necessary. This task is typically managed by Implied Logic in the hosted context.