If it's in the hundreds of milliamps, that may be the problem if the battery runs down in just a couple weeks. A poorly designed or installed car alarm system may have the same effect. You can check the standby power drain on the car by inserting an multimeter with a sufficient DC ...
Remove the positive battery cable. Set the multimeter to the largest amp setting. Connect the negative probe on the multimeter to battery cable you just took off. Connect the positive probe to the battery positive (+) post. Gradually reduce the multimeter setting of the lowest milliamp setting....