Originally Posted by FormulaMMM
Pad deposits. I experienced the same on my old M5 and with the M3 until I smartened up.
If tracking is going to be a regular thing, strongly encourage you to get at least a front BBK and legitimate track pad (Pagid).
If you will only do a handful of track days a year for fun, get some racing pads like Pagid, Hawk, or PFC. Also upgrade your fluid.
If you are going to be a track regular, go directly to a BBK.
Let me know if I may be of help.