switch off my car and undo the s? Hi!

I have a 2000 toyota sienna minivan LE, recently i noticed that when i switch off my car and undo the seat belt, there is a noise like (tick tick tick), sound like electrical and it continues even though i have taken out the keys, it is killing my batterry to a level that next day i could not start my car and non of the odometer lights comes on. I jump started my car and took it to mechanic and he could not figure out as well. Now i have changed the battery and same tick tick comes back even if the car is off. However, when i drive my car and wear the seat belt, the noise goes away. Also, my keyless entry does not work either, this all leads me to believe there is a problem with electrical. If so, should i go to the dealer or there is an easy fix that i can do.

please advise.

thanks