My `87 Corvette was doing that. I did not drive it for a few weeks & BAM! dead battery. I now keep it on a battery tender---- Works like a charm & the best part is, you can get a cheap one for under $10.00 from Harbor Freight

 

There are only a few things that can cause parasitic battery drain:

The battery itself - test it properly or have it properly tested.

The charging system - alternator output can test OK (13-15V output), but a bad diode in the alternator can cause a drain. Leave the car off and the battery connected for a while then touch the alternator. Is it warm or hot? Replace it. This was my 68's problem. I replaced the regulator too for good measure.

A short or open circuit in the system - this the nasty one and hardest to trace. If battery and alternator and known to be in proper working order, disconnect the positive battery cable and connect a volt meter to the positive battery terminal and the disconnected positive cable. Depending on your car accessories there should no drain or only be a couple of volts at most. If you know how, check the amp draw. It should be under 50 or so, but I could be wrong on that - don't remember. Check your manual for proper current draw results when the car is off. If you believe it's short, then one by one start pulling fuses to identify the circuit with the problem. Then check all the wiring for problems.

Typical items - glove box light stays on, trunk light stays on, after-market stereo incorrectly connected or stereo malfunctioning, recent body repair crimped a wire, recent replacment of electrical equipment or add-on of electrical accessories has a shorted wire somewhere.

Good luck!

 

mine was too, then found out bad alternator