A lot of things going on here.

I know that the Orange trail on Sureau is maintained by SCT, so there's no problem.  In Flagg Hill (SCC), the posted rules do not prohibit hiking on trails not shown on the official map; the prohibition is for bicycles and horses.   It is not even prohibited to walk off trail.

I will make a note to check bicycle/horse access tagging vs the SCC-published map.  I've already done that for Town Forest and Captain Sargent.

If you find a place where a trail goes onto private (vs SCC/SCT) land, and it's posted no trespassing but not marked access=private, or it's on residential land and not marked access=private, by all means open a note and state that clearly.