Inspection of code for the $exists matcher suggests it may treat a missing field and an existing field with a null value identially, which is not how the server $exists operator works. This ticket is to:

Ensure there is test coverage for an $exists match on a field explicitly set to null
Fix the matcher if necessary to have it behave identically to the server
Test documents with arrays which satisfy $exists: true and $exists: false concurrently (for subqueries)