I would get some BMW coolant and top it off. Up there I would run about 65% coolant to lower the low temperature safety point for freezing.
Be careful if you are going to keep coolant in the trunk. If it spills and your MPM hasn't been moved from the normal position under the spare tire you will have a lot more to worry about than a low coolant light. If you decide to carry the coolant I would enclose it in a 6mil or 7mil heavy duty plastic bag, which auto parts suppliers use to ship gallon jugs of coolant via UPS or USPS.
And if your cooling system hasn't been flushed and filled with new coolant in the last 3 years that should be on your maintenance list for this summer.