Mmm something doesn't add up, no?
The 172 cup has a shift light, but not a shift beep (that was the 197 onwards IIRC) as far as I was aware?!?! The shift light is a yellow lorry-looking gearstick at the bottom right of the dash should. It should also never come on anywhere massively below ~6800 RPM (the lower rev limit set by the ECU for when the engine is not upto working temperature)
The shift light should work whether you are driving or sat in nuetral.
As silly as it sounds I don't think the light you're seeing is the shift light. Have a look in the owners manual. My guess is it's the emissions light?