Here is how you can tell if you have a drain, the cheap and easy way.
1: Go buy a 12 volt test light.
2: Take the bulb out of your dome light. This allows you to leave the door open for step 5.
3: Disconnect the negative wire from the battery.
4: Attach one end (doesn't matter which) of the test light to the battery and the other to the negative battery wire. In other words, you are completing the circuit with the test light. Don't try to start the engine or turn things on as it will probably blow the test light's bulb.
5: With the key in your pocket and the engine and everything else in the off position (radio, fan switch, wipers etc), if the light comes on, you have a constant draw on the battery. To find out what is causing that, pull the fuses
one at a time until you find one that makes the test light go out. Your problem lies on that circuit.
If after performing the above tests you are still unable to find out what is killing the battery, I would suggest putting your high beam switch in the high beam position (pushed towards the dash with the headlight switch turned off) and leave it there for a few days and see if you still have the problem. I've had two Nissan pickups act the very same way with me and the multifunction switch turned out to be the culprit both times. The only way I figured out what it was was because a neighbor called me and asked me if I knew that the headlights were on in my truck.

Good luck
Jeff