Funny you should just add this now. So I was in a PT today trying to get a coffer key for a lsmate. So I fall down to hume bones and turn right, and look, I see a coffer. Being the thiefly person that I am, I attempt to open it as a 64THF with living keys, since he didn't have his coffer key yet anyway and somebody else would probably open it first. Attempt. Failed. Try again. Mimic!
/p oh crap i'm gonna die guys help
They came, killed it with ease (75mnk, 73rdm, 56war, 72pld, and me). Wasn't even a challenge, so these aren't too hard, but hard enough that I'd have probably not been able to solo it yet. 1/1 on key. I think either Mimics have 100% coffer key drop rate or pretty close to.
Coffer instantly repopped on OTHER side of hume bones. WAR got his old guantlets. That was the shortest coffer pt I've ever had.