SQL Server 2008 R2 "Ghost Memory"?

You won't get a true picture of memory usage from Task Manager if the account the service is running under has the lock pages in memory privilege (edit: as per Mark Rasmussen's comment/link). To determine how much memory is being used you can look at:

• SQLServer:Memory Manager\Total Server Memory perfmon counter
• DMVs

I can't recall if there is a DMV or combination of that will give you the total memory allocation but the following will show the bulk of it.

SELECT TOP(10) [type] AS [Memory Clerk Type], SUM(single_pages_kb) AS [SPA Mem, Kb] 
FROM sys.dm_os_memory_clerks 
GROUP BY [type]  
ORDER BY SUM(single_pages_kb) DESC OPTION (RECOMPILE);

SELECT DB_NAME(database_id) AS [Database Name],
COUNT(*) * 8/1024.0 AS [Cached Size (MB)]
FROM sys.dm_os_buffer_descriptors
WHERE database_id > 4 -- system databases
AND database_id <> 32767 -- ResourceDB
GROUP BY DB_NAME(database_id)
ORDER BY [Cached Size (MB)] DESC OPTION (RECOMPILE);

The second is the most interesting usually, buffer pool allocations by database. This is where the lions share will be used and it can be useful to understand which of your databases are the biggest consumers.

There's a recent article from our own Brent Ozar that treats this case, when Task Manager doesn't show correctly the memory eaten by SQLServer and its additional services. You can find it here: A Sysadmin’s Guide to Microsoft SQL Server Memory.

Quote: "Why Isn’t SQLServer.exe Using Much Memory?

When you remote desktop into a server and look at Task Manager, sqlservr.exe’s Mem Usage always seems wacky. That’s not SQL Server’s fault. Task Manager is a dirty, filthy liar. (I know, it sounds like the SQL guy is shifting the blame, but bear with me for a second.) On 64-bit boxes, this number is somewhat more accurate, but on 32-bit boxes, it’s just completely off-base. To truly get an accurate picture of how much memory SQL Server is using, you need a tool like Process Explorer, and you need to identify all of SQL Server’s processes. In the server I’m showing at right, there’s two SQL Server instances (shown by sqlservr.exe), plus SQL Agent, SQL Browser, and SQL Server backup tools. It’s not unusual to also see SQL Server Analysis Services, Integration Services, and Reporting Services also running on the same server – all of which consume memory.

So how much memory is SQL using? I’ll make this easy for you. SQL Server is using all of the memory. Period."

So I'd advise you to try Mark's query and use a better tool for memory report. Or just trust Perfmon to report memory, not Task Manager.