1999 Lexus - when I don't use car 4 several days it is dead

if the battery is new and the alternator has been changed there must be some form of parasitic draw on the battery. Some electrical load is staying on after the keys have been taken out of the ignition switch. could the interior light or trunk light be staying on? To find out if there is something staying on that would be running down the battery the battery is disconnected and an amp meter is put in the circuit to see if the battery has a draw. Disconnecting the fuses one at a time while watching the amp meter would determine which circuit is causing the electrical draw.

The first answer provided is correct. There must be a draw on the battery that is not apparent. I had this problem with my 2005 Lexus ES 330. I reached the point where I had a supplemental battery charger that I hooked into when I put it in the garage. I took it to the dealer here, who replaced the battery a couple of times, and of course ran extensive and expensive tests. I had purchased the vehicle from another dealer with about 10,000 miles on it; they used it to provide a car for their customers while the customer's care was in repair. The dealer here -- after I spent hundreds of dollars with them searching for an answer --discovered that the dealer from whom I had purchased the car had installed a gadget that was designed to track the car in case it wasn't returned by a customer. They forgot about it, and left it on when I purchased it. It was drawing down power and that was the problem. Once that was removed, I've not had a recurrence.