Join Date: Dec 2004
Location: Altofts zx-12r B4 UK
Here are a couple of possibilities, the start button is stuck on, the starter solenoid is stuck on, the starter relay is stuck on, and there is the chance of a short somewhere.
If it was ok when you picketed it up and all you've done is take the battery out to charge it up then chances are its something you've done in the battery tray.
Here is the starter system diagram for your bike.
For Tech Questions Bike Model letters and Country are more useful than saying the year, for UK owners anyway, as registration year is not always the year of manufacturer model.