Case ( IsEmpty ( FilterValues ( List ( Leases::Expired ); "No" ) ); "Yes"; "No" )
However, I think that you may be handling this the wrong way. I think that you should probably just sort descending by lease date or lease end date. If the lease end date is past the current date ( plus a buffer for turn over if you wish ) then it would be considered "available". No need to mark each Lease record.
I thought marking each record would make it easier to manipulate the data, ie easier to create filters.