Query to compare 2 collections and find one different record

I have 2 collections in my database that have one thing in common: - _id (ObjectId). I would like to get one record that is in one collection but not in the other.
One collection has 500 records.
The second collection has approximately 15,000 records.
There are 501 records in the second collection that have the status “ALIVE”. 500 of those 501s have _ids like the first one, one is ALIVE (rest have got SLEEP status), and is not in the first collection, but it should be. I’m trying to track her down.
To sum up :
Query should compare _id in collection no1 with _id + ALIVE status records in collection no2 and find the one ALIVE that is not in the first collection.

We could help you better if you provide sample documents from both collections and sample result.

To come up with a solution, we need to insert documents into our test environment. Working with real documents is a lot simpler for us. And for you supplying sample documents should be trivial.