That certainly was not a Ninja climb.  
Nearly all the municipal parks that I know of will allow arborist get-togethers, and their related activities.  All with due process of course.  Proof of insurance is the biggest.  
Now, going out on your own, in a municipal park, in broad daylight, is going to attract attention like what you encountered.  Rarely does it not.
As for my climbing in the State and National parks, 30-40 years ago, I always keep a low profile.  Hike in early, don't draw attention, get out without a trace.  Out of many such climbs I was cited twice and advised on a couple other occasions.  One incident turn hostile.  
For the times I was discovered all such incidences happened only because of who I was with: being either noisy in the trees, bragging to others about what we were doing, or inviting a whole slew of their friends to come and watch, which for me soured the whole experience.    
My best climbs in the old-growth where always the ones by myself.  When you're alone, without any distractions, you truly become part of it.   It's almost a religious experience for me.