checked the pressures.... all tires are lower than normal, but the front left one (which was indicated by the TPMS) is even lower, at 22.5. So, i think it's an effect of the cold weather in general. I will fill it up later.

BUT... this is just ONE reason that BMW should have SHOWED us the actual pressures in the iDrive system, rather than just a warning. It would have been much easier to figure out, rather than having to get out and check the pressure manually.