I am not expert about this, but sure it is deficiency for me. it a part of pre-flowering stage that plant will use more nutes for stretching at later. are you giving her cal-mag ? im not sure what is your plant problem so may need someone who know to answering this. in my grow I use to give cal-mag when pre-flowering about to start.
Talk about pre-flowering in autos, I tend to count it when I saw those small Calyx start to develop at nodes. it may not have hair at first but if you close up monitoring you will see this sting at every nodes, after she enter pre-flowering small calyx will appear. this is from my plant.
another thing that might cause this happen is your pH water it is important for plant to intake specific nutes from what pH she take. and your soil look like too compact to me. how is it dry out ? but your plant look good beside from the sign of deficiency.
this is what I learn from pH effect nutes intake, pH 6.5 is sweet spot for soil.
As for timing, most Autos go into pre-flowering around week3-5. but poor condition can cause it to delayed. I myself facing this problem also. just start preflowering at day 35 for my girl. so no worry.