Respected Ishna ji,
I quickly searched SGGS and found a shabad on ang 727 (attached file)... though I understand it may be irrelevant to your query.
Turban is important for a few reasons:
1. Our Guru wore it
2. Guru ordered sikhs to carry ourselves with higher self esteem (Kings used to wear turban that time)
3. Guru wanted sikhs to stand out for easier recognition
4. to maintain hygiene of kesh
Just like there is no direct mention of cleaning teeth, using facial creams, jogging, going to work in cars, flying by planes etc etc, there may not be mention of turban!
Please forgive my ignorance.
With Regards,
Arvind.