Noosignatures were proposed to fill the gap between biosignatures (chemical signs of life) and technosignatures (detectable advanced technology like radio signals). They indicate intelligent activity that is not necessarily technological. Examples include:

• Stone tools (e.g., Acheulean hand axes) 
• Architecture or carvings
• Undeciphered writing systems (e.g., Indus Valley script) 
• Complex cultural products such as bread, used as a thought experiment for measurable complexity 

Chemical signs of possible life go back billions of years on earth. Production of radio waves, a bit over a hundred. Noosignatures on earth would go back maybe 3 million years or so. Exactly how you could detect these from earth is unclear.

The “noo” in noosignatures is supposed to be pronounced like the “oo” in “noon”, though.