Where is the idol. I've looked every where in Martens's Bluff and yet I haven't
found it! I'm guessing that it is behind this door that won't open near the T'rang trap. Help!

If you're talking about the courtyard door opposite the very elaborate slamming-walls trap, then you are correct. However the door is controlled from a room upstairs in the bluff -- you'll need to pull a lever to unlock the door, and there's no indication in the area that the two are connected.

Spoilor
<
<
<
<
<
<
<
<
<
<
<
To get through the door you have to start at the mines and go through a passage to get to a section of bluff that's not accessable by any other method. There you will find a lever that will open the door.

Spoiler
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
If you have a Pulse Pick (there are two in the game), then you can get Marten's Idol in a different way.

Another (not recommended) possibility is to fight the T'rang that won't allow you to enter certain areas (but will make the T'Rang permanently hostile)

Another way is to combat run past the restricted areas. They will warn you to stand back again once you are past them, but they do not turn hostile.