There is no way for me to know how it was tested as every tester is different but 15 volts is a bit odd as the regulated charge volts should only be from 13.5-14.8 volts. In any case it sounds like they passed the alternator both times
The problem I have is.. it will pass each time UNLESS you take it in and they run it on tester long enought till the problem happens. You said its unpredictable. Each time you have taken in it may have been just fine. having light come on, remove alt, get to tester? yes.. it very well will test ok each time. Intermittants are very hard to find sometimes
The thing to do is do your own testing. get a cheap Volt meter for 12 volts and the next time light comes on, touch the battery terminals with the leads and you should see around 14.8 volts. If out of this range,, then you most likely have an alternator failing or more so the internal regulator.
It is very rare for the cluster or wiring on a vehicle this new to cause the problem but cant rule out. You must catch it in the act with meter on the battery terminals. You either have around 14.8 volts or do not. IF light is on and you test and it IS around 14.8 volts, then you have a problem within the instrument cluster most likely
I am leaning to bad alternator. Just recheck all connections to really be sure. be sure all clean and tight
here is a symptom chart and you will see on left column the symptoms and right column the possible causes.