I know you said that you are parking in the middle of a parking garage, but if the spot gets sun at all, you could think about a solar charger. My old VW has one that plugs into the cigarette lighter. If you haven't figured out your car's battery leaking problem, you probably won't be able to hook up a battery disconnect switch. They DO however, have automatic disconnects that disconnect the battery if it senses a battery drain (and you can still keep your radio settings.) Personally I'd just disconnect the battery every time you park it, it only takes 20 seconds and you can keep the 10mm wrench in the car. When you come back, just buy yourself one of those portable jump boxes. They're heavy as heck, so you'd have to find a way to transport it on your bike however.

Anyway, if the car does get sun make sure you use a sun shade. Also, if your tires get sun, make sure you cover them. UV damage is the largest cause for tire degradation on not-often-used cars/trailers. A piece of cardboard will work just fine.

Other than that, don't worry about it. My car sits under a cover for 4-5 months every winter, and it's fine every spring. (I do remove the battery however.)

Finding what's causing the drain is easy. Hook a multimeter up between the battery terminal and the thing that usually sits ON the battery terminal. You want it on the amps setting, probably the 10A setting at first to be safe. Once you see that the current is < 10 amps, you can use the lower current setting (usually 400mA) So, disconnect one side from the battery, hook up the multimeter in between the battery and the wire. (12V cannot hurt you, don't be afraid.) Then go to your fuse box and start pulling fuses. DO NOT TRY TO START YOUR CAR. You will destroy your multimeter. Once you pull a fuse the significantly reduces the current being drawn, you know that's your problem. I think anything less than 100mA is normal for a newish car. My old 93 car draws 40-100mA and it sits for relatively long periods just fine.