I work with several PowerBuilder developers and have the same issue. It is generally an open transaction on the tempdb. Tempdb will usually check point after I kill the open tran. I also use this script to quickly check free space.
SELECT DB_NAME() AS DbName,
name AS FileName,
size/128.0 AS CurrentSizeMB,
size/128.0 - CAST(FILEPROPERTY(name, 'SpaceUsed') AS INT)/128.0 AS FreeSpaceMB