Sql-server – Find when a specific database went offline and the reason for same

I initially thought you were on to something here. Working assumption was along the lines that perhaps the buffer pool wasn't immediately flushed as it requires "some work" to do so and why bother until the memory was required. But...

Your test is flawed.

What you're seeing in the buffer pool is the pages read as a result of re-attaching the database, not the remains of the previous instance of the database.

And we can see that the buffer pool was not totally blown away by the detach/attach. Seems like my buddy was wrong. Does anyone disagree or have a better argument?

Yes. You're interpreting physical reads 0 as meaning there were not any physical reads

Table 'DatabaseLog'. Scan count 1, logical reads 782, physical reads 0, read-ahead reads 768, lob logical reads 94, lob physical reads 4, lob read-ahead reads 24.

As described on Craig Freedman's blog the sequential read ahead mechanism tries to ensure that pages are in memory before they're requested by the query processor, which is why you see zero or a lower than expected physical read count reported.

When SQL Server performs a sequential scan of a large table, the storage engine initiates the read ahead mechanism to ensure that pages are in memory and ready to scan before they are needed by the query processor. The read ahead mechanism tries to stay 500 pages ahead of the scan.

None of the pages required to satisfy your query were in memory until read-ahead put them there.

As to why online/offline results in a different buffer pool profile warrants a little more idle investigation. @MarkSRasmussen might be able to help us out with that next time he visits.

One suggestion would be to identify the index that is causing the problem.

select o.name as tablename, i.name as indexname 
  from sys.objects o 
  join sys.indexes i 
    on o.object_id = i.object_id 
  where i.index_id = 7 
    and o.object_id = 1253579504

and then drop it. If you believe you're going to need this index then I'd script out the DDL before dropping it, drop it, recreate it, and then rerun your DBCC. Since its index_id is 7 I'm going to assume that this table is potentially over-indexed so YMMV with actually recreating it.

If dropping the index doesn't work, then I would always recommend a last-known-good backup to which you can restore. Also, if this has been occurring for a while, the backup will also have the corrupted index since a full backup is a page-by-page copy of the database (+ some transaction log).