Blonde patch of hair?

My son is 14 days old, & today I noticed a blonde patch of hair on the crown of his head.

I'm 99% sure he wasn't born with it, but he has been in the NICU so 99% of the time I'm the one holding him so I haven't really had a good look, but no one has mentioned it before so I don't know 🤷🏼‍♀️

I'm shocked at how light it is! My first son is white blonde, but he has a different dad (my current partner has dark hair but was born blonde).

I'm just wondering if anyone has experienced this is has any idea what us going on?