File not found: file is in ~/Library but not on 'This Mac'
Spotlight is intended to let users find their own files; since the Library folder normally contains files the user doesn't need to (and indeed shouldn't) access manually, it's left out of the search results (along with other "system files"). You can find files in it by either searching it explicitly (as you mention), or by telling spotlight to include system files in its results (see Apple's KB article #HT4355 -- note that it only mentions OS X 10.6, but the same procedure works in 10.7 and 10.8 as well).