A windscreen crack appearing without any obvious impact damage is likely to be caused by excessive stress, whether that is a particularly sharp pothole impact or extremes of temperature. It is unlikely that this will be covered under your warranty unless it can be proven that the windscreen was defective in some way, but if you feel this is the case then we would suggest having the screen inspected by your dealer. However, the most likely scenario is that the dealer will say it has failed due to stress and quote for a replacement. A better option may be to go through your car insurance provider if your policy includes windscreen replacement, as you will only have the pay the excess and it will not affect your no claims bonus.