Exactly.
1. All switches default to one VLAN across all ports, almost (?) always VLAN1.
2. Nope but this is the reason some people don't trust VLANs for separating network segments especially, say, WAN and LAN. That and the possibility that your switch firmware has some exploitable bug allowing packets to change VLAN, never seen that either.
3. Some switches have an unmanaged mode they default to that is indicated somehow. The Dell PowerConnect range, for example, have a managed mode LED on the front that tells you the switch has been configured away from it's default state. If it did default back to unmanaged mode the LED would go out, you would know.
Steve