As recently posted on another thread, an old mate who works for insurance companies tracing stolen cars tells me there is a method to locate OE trackers (which was not divulged). The advice if your motor is special to you, is to fit a second, non BMW tracker. Apparently this is foxing the thieves since they stop looking once they find the BMW tracker and some motors have been recovered.
In the 'old days' we used to rig our cars with a hidden or disguised cut out switch in the LT ignition circuit, an unused button on the dash was favourite, but I don't know if the electronics would let you do it now. Not high tech' but it worked.
See the previous thread re hacking in to cars with laptops, particularly BMWs, this should be well known to Police stolen vehicle specialists (but not necessarily mainstream cops). I assume you are happy it was secured? if you have problems convincing the Police that it's possible to steal a secured BMW without keys PM me and I can put them in touch with the guy I mentioned above, his job is sponsored by the car insurance industry and he works closely with the Police.
http://www.z4-forum.com/forum/viewtopic.php?f=5&t=32764&p=455825&hilit=+tracker#p455825
(edited with working link)