I have some brown spots that I can't seem to identify

[thegrower1]

started 4th week of flowering. I'm running a hydroponic dripping system on expanded clay , ph is 5.8-6.2 EC is 1.6 using mostly bio-nova.
the past week these brown spots started showing on the fan leaves all over the plant, is some places more pronounced than others.
I thought it to be cal or Mg problems, but it doesn't seem that way. could be a fungi, but my tent humidity was 20%-30% and temp is in the 25c-29c range the whole grow time so I am not sure about that.
strain is an automatic indica

 

[BigCube]

It almost looks like rust (a fungal growth) but it could also be excess pk.

 

[LJ66]

Have you checked for pests? It’s hard to see it properly but spider mite damage can look a bit like that.