In regards to the '500k' mystery blueprint writing.
Keep in mind all this is written on a TPI map, so it was written by trevor himself, so the note you see on the back, was a note to himself so he could remember where to go for a certain drop off.
People saying its the submarine docks, I doubt it, why write something on a blueprint map that you can see clearly from the original in game pause menu.
I suggest using trevor to investigate this as it was probably a note to himself on this map.
When the guard rail ends on the correct side of the road coming from the south, the steel rail turns into a wood one, and then splits of into a dirt road, follow it and it leads down to the rocks at the sea line. And then further down onto the beach.
I have a feeling this will be nothing, just like the morse code hatch was.