Haven't used what you're describing; assuming it will operate just like the Find method, i.e. it will only find the 1st record that satisfies the criteria. You need to include the condition where the check in date is Null for an ID, so you cannot simply based this on a combo selection, unless you include the record id (unique, such as autonumber) in the combo's list, and ensure the combo only returns list items for an ID where there is no check in date.
If your names really are as posted, suggest NO spaces or special characters (save for perhaps underscore) http://access.mvps.org/access/general/gen0012.htm
and NO reserved names such as "Date" One source about how to name things http://allenbrowne.com/AppIssueBadWord.html
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.