If getting to work is critical you need to take precautions as previously suggested. In terms of will it flatten your battery. It is impossible to say but there is a good possibility the state of charge of your battery will be insufficient to start the car, the battery being in the boot doesn't help you much either.
To do some maths.
An indicator bulb is 21W @ 12V. Assuming your battery has a terminal voltage of 12V it will draw 1.75A which means every hour your battery capacity will reduce by 1.75 Ah. I am not sure what the quiescent current (the current taken by all the permanently connected electronic modules while the car just sits there) of a Z4 is but assume 100mA worse case. The drain on your battery will be 1.85 Ah per hour, so after 12 hours the capacity will be reduced by around 22Ah.
According to Real OEM the battery sizes on a Z4 range from 55Ah to 80Ah. So assuming your battery is in tip top condition and 100% charged a 55Ah battery would drop to 33 Ah. But it is unlikely that your battery will be in such condition, over time the active material of the battery degrades according to how well it has been looked after and its capacity to hold charge will go down.
Z4 3.0L 2005. Titan Silver. Audio Upgrade Second Stereo Pioneer X8700DAB, Sony Xplode 4 x 70W Amp, 6.5" Reckhorn Subs in rear cubby, Focal AS165-3 in fronts.
