This is why it is advantageous to only say the word "no" and use a stern voice when you really mean it. I save it for health & safety concerns, personally.
You can also try using something other than "no", like "uh-huh" or whatever is a noise you don't normally make for the non-safety/non-emergency scenarios.
Use a 'teacher' voice, and move her away from whatever she's getting into.
All that being said, make sure your house is baby/toddler-proofed as much as possible to prevent as much as possible.