It will stay exiled.
You apply Tawnos's Coffin's effect part by part. When you have finished applying the first part (“Exile target creature and all Auras attached to it. Note the number and kind of counters that were on that creature”), Tawnos's Coffin is already in exile, but the delayed triggered ability that's meant to bring it back doesn't exist yet. Since that trigger is created later, it will just never have an opportunity to trigger.
Now, you will say, “but I've learned to read the whole of a spell or ability before applying it!” You're right, this rule exists, but it's meant to cover specifically those cases where the spell or ability doesn't make sense when applied sentence by sentence. There's two typical cases for this:
1) The second part replaces what the first part does. These are called self-replacement effects; the canonical example is
Brimstone Volley. Applying the second part on its own doesn't make sense, you need to know the second part at the moment you apply the first.
2) The second part specifies something about the first part, for example
Terror. The second part, “it can't be regenerated” doesn't make sense if you destroy the creature first; so you have to know the second part in order to apply the first part correctly.
Tawnos's Coffin doesn't fall into this since both parts make sense independently of each other (other than the fact that the two parts are linked and the delayed trigger will know what the first part exiled), so you just apply the first part and then proceed to the second.
Daniel Kitachewsky
L3, Paris, France
Rules NetRep