X5-2007-3.0si-160K KM

Have a recurrent issue; every couple of days I get a strong smell from the vents, it is a bit of burning smell (it does not smell like oil, not electrical either). The smell is specially pronounced after breaking hard or when starting the car just after a long drive.

I believe is a Coolant Leak as I had to top up the tank significantly, there are no signs on the floor and when I have opened the hood to check for a 'smoking' source have been unable to see anything.

Any suggestions on what to look for / where to check?

Thanks