Lets write a function that determines if there is an obstacle in the way. This indicates where, if any, the object might be. The Tips Build yourself a little IR detection tool. This requires a Sharp GP1U5X, a visible LED, and a resistor. With this little tool, you can check to see if your IR detector is actually emitting IR or not.
Black plastic is a good example.
Many forms of black plastic are transparent to IR light, and therefore doesn't make a good shielding material. I would suggest using aluminum foil covered with electrical tape as a good shielding material. I have had good luck with it. Most consumer video cameras can see IR light. This is really useful, since you can aim your video camera at the robot, and see the emitted IR light.