If it makes you feel any better, I added 1 3 weeks ago and haven't seen it since.
I also added a reef-lobster about a month and a half ago, never saw it again, until last week.
Mandarins feed on pods, which mainly come out at night. Have you tried the red-LED flashlight in the middle of the night yet?
If you don't find the madarin you may be suprised at what you do find.