It is normal for a battery to go flat on one of these things, especially after a month and a half, as some devices still remain active, like the immobilizer for instance.
If you couldn't get it open via the manual way, try the other front door, as sometimes it'll work on one side but not the other.
If your battery was going flat on a regular basis, like if you let it sit for a day, then you might want to replace the ignition switch as that is usually the common culprit for battery drain. A failed switch won't shut off properly leaving more devices on than it should.
As for your plastic key, not completely sure what that is...