It sounds like you are using the Nest Protect, at first I thought you were using the new Nest Alarms device.
I have 3 Nest Protects, and of all the Nest devices I am least happy with it. I too get an occasional "smoke in Basement" notification, however it never goes to a full audible alarm, just the notification. Try clearing the sensor chamber with some compressed air in a can. I assume there is no longer steam or smoke present.