FWIW, 32767 is 2^32-1 or hex 0xFFFFFFFF or -1 in two's compliment. This tells the software engineer in me that altitude is stored and displayed as a 32 bit unsigned integer, but somewhere else if somehow altitude is not available or there is an error in calculating it, the signed integer value -1 is returned as an error value (quite common in software engineering).
Or maybe you really are 1m below sea level. Or maybe your GPS calculated altitude really is 1m below sea level...
I was parked near my home at about 700m above sea level. I assumed the satellite signals to calculate altitude and location were not all acquired yet. In a few seconds it went back to normal !
Another possibility is the existence of an official armoured vehicle or building which have powerful frequency inhibitors. These were designed and very common when terrorists used to detonate bombs by remote control using receivers or mobile phones. When driving near one of these, the car also advises that the key is not found.