Default behavior:
db.grep("search term")
OR
db.grep(ObjectId("56e69b8abd177faa0aab0bfd"))
Goes through all indexed fields in all collections and lists all collections where matches exist

Quick way to test:
https://gist.github.com/blowhacker/c930142e967d3a123519
copy the contents of this gist into a mongo shell and try db.grep( <something to grep>)