that's impossible. they can't track that you're using bluetooth for audio out, and it has nothing to do with tethering. the CSR is mistaken. someting else triggered them.
Nexus 5 - CM11 / Android L dual-boot
Nexus 7 - CM11
Samsung Galaxy S III - CM 10.2 Jelly Bean 4.3
Motorola Atrix 4G - CM9 Ice Cream Sandwich 4.0.4