These were recurring calendar entries the user deleted. The reason for it is that the CMT Universal migrates recurring meetings and appointments based on the parent document, and doesn't read the child documents. Changes to meetings are added to the child documents, but not to the parent. So when the child is deleted, the parent is not updated. The migration process migrates the parent document, but can not find the child. This is why you received the error reported below, and this is why the item was added to the deleted items folder. The migration realized this child entry was deleted.
I've included documentation regarding repeating meetings.
CMT for Notes (“CMT Classic” or “CMT”): Conversion process for Repeating Meetings
CMT converts Outlook repeating meetings by getting the start date and time, end date and time and the repeating pattern and number of occurrences from the hidden binary field. The hidden binary field is undocumented by Microsoft, so little is actually known about the format. The resulting information is converted into a series of dates which represents the pattern. Unfortunately, the exceptions pattern entries are not currently processed because we don’t know the structure of the format. Therefore, we can not recreate the exceptions to the original pattern and the resulting calendar entries that appear on the user’s calendar will not always match.
CMT creates a parent document containing all of the information about the repeating appointment to include start date and time, end date and time, repeat dates and all necessary repeating type fields which are used to make up the actual pattern. CMT also creates child documents to represent the actual calendar entries which appear on the user’s calendar.
CMT does not support work flow because of the difference between the systems. Outlook uses a random 9 digit number to associate repeating meetings between the Chair user’s mailbox and the Invitee users mailboxes. Lotus Notes uses a Universal Note ID which actually represents a document in a database and is therefore not random.
NOTE: Outlook supports repeating meetings with no end date. Lotus Notes does not support this type of repeating meeting so meetings with no end date are assigned a number of occurrences based on a configuration setting which is defaulted to 10.