Can SQL Server 2014 server reported drive space properties change overnight when database growth is set to Unlimited?
I ran the following query accessing the Default Trace log to find when the database did an Autogrow:

DECLARE @path NVARCHAR(260);
SELECT 
   @path = REVERSE(SUBSTRING(REVERSE([path]), 
   CHARINDEX(CHAR(92), REVERSE([path])), 260)) + N'log.trc'
FROM    sys.traces
WHERE   is_default = 1
SELECT 
   DatabaseName,
   HostName,
   ApplicationName,
   [FileName],
   SPID,
   Duration,
   StartTime,
   EndTime,
   FileType = CASE EventClass WHEN 92 THEN 'Data' ELSE 'Log' END
FROM sys.fn_trace_gettable(@path, DEFAULT)
WHERE EventClass IN (92,93)
ORDER BY StartTime DESC


This showed the database autogrew by 10% during the time the change in drive space was noticed.  So the database simply did what it was supposed to do: grow by 10% when it was needed.