Is there a guaranteed pin level after calling noTone()?
Can I be sure it always stops at LOW? Or it can stop anywhere and I have to set the pin LOW manually after noTone()?
I am driving transistor by the pin and stopping at HIGH can damage the speaker or cause high current issues.
Why not do a digitalWrite(tonePin, LOW); right after noTone().
The documentation does not specify it: noTone() - Arduino Reference
The "Tone.cpp" for the AVR microcontrollers shows that the pin is kept as output and the output is set to LOW (they use 0, but it should be called LOW).
It does not wait for the signal to be low when noTone is called. The signal is stopped and made low at that moment.
Since it is not documented, there is no guarantee. I suggest to make it LOW to be sure.
(While I was typing this, groundFungus already wrote the same).