I have a 2001 Tribute V-6 and the battery will discharge on occasion for no apparent reason. It will go dead after just sitting in the garage for a day or two with nothing left on. It can be fine for awhile then its just dead. I have had the battery tested at 4 places and it checks out good. Something seems to be draining the battery but I have been unable to find the culprit. I would appreciate any advice on this problem.

 

First, all modern vehicle have a "phantom power" draw. This is to keep things like radio, clock and remote locks/intrusion alert settings, as well as information stored in the ECU/PCM from being lost.
Since you seem to have a higher than average draw, I must first question you about any added devices installed, such as: An aftermarket alarm system, or remote start system, aftermarket stereo/video, a PA system, etc? These things are always suspect, and usually the cause of a problem such as you describe. Sometimes they were not installed correctly in the first place. Other times, something has gone wrong with them and they are shorting out.

Either way, you must first connect an ammeter in-line with the battery. If you don't know already, this will read the amperage being drawn away from the battery when the car is off.
Begin disconnecting any such equipment to determine where the draw might be. One at a time disconnect every thing you can think of, then leave them disconnected until the problem is located. Often, you can do this at the fuse box(es) for convenience, but if the draw persists, you must find the power leads and disconnect those.

 

Sometimes a diode in the alternator will cause the battery to go dead.
After you drive the car wait a couple of hours and feel of the Alt. If it is warm you may have found your problem.
You do have to wait long enough for the engine and all under the hood to cool off.