Plumbing by-passes or abandoned lines; collapsing sewer lines; badly corroded supply and waste lines (both can be from inside or outside corrosion) fixtures with very hard to find components; hidden electrical junction boxes...or non-existent boxes at junctions; outdated and/or underrated distribution panels; knob and tube wiring improperly connected to, or extended with, romex; Switches wired incorrectly, breaking the neutral instead of the hot side; no, or poor grounds for improper outlets; overheated connections, outlets, switches, and light fixtures; improper wire sizes for circuits and overloaded circuits; and plenty of others...all very costly to troubleshoot, locate, and correct individually.

There is another similar valve upper left above the hot water tank that also has a slow leak, but the lighting was poor so you can't see it:Closeup of the leak:Closeup of the other leak (this is the valve above the hot water tank that you couldn't see in pic 1):Also noticed this corrosion on a completely different section. 

See manufacturer’s specifications.)7.Location of Fasteners: Corrosion resistant fasteners shall be located per manufacturer’s instructions and not placed in factory applied adhesive.

This could be zinc oxide from galvanized pipes corrosion.

Inspectors for two of the properties told the buyers that new water heaters were necessary only because there was  very little corrosion on the water pipe nipples on top of the water heaters.Just last week, one inspector made be get mold testing because he said a black mark on the door where the door was closing against a 2 inch x 1-1/3 inch portion of the door appeared to be mold.

Galvanized lines should be replaced - if they are not already constricted to a trickle due to internal corrosion, they will be.  

It looks rusted right where it comes out of the ground in the crawl space but it hard to know how much corrosion inside - it's probably the original 100 yr old pipe, so I'm guessing replace as well?

this is the scope of work. the dimension of the roof is roughly 45X151) Tear-off all layers of roofing and gravel down to wood decking and legally dispose. 2) Inspect wood decking and replace all deteriorated and broken areas at $ 2.50 per sq. ft. 3) Inspect existing Ceramic Copings and replace all deteriorated and broken at $ 35.00 Each.4) Install Fiber Cant Strip at the base of the walls and curbs.5) Install 1 layer of Base Sheet over wood decking using corrosion resistant screws and plats. 6) Install 1 layer of Modified Bitumen CertainTeed GTA Granulated. 7) Install Metal gravelstop and gutter flashings at the roof edges. 8) Flash all vents, soilpipes, and chimney using the same Modified Bitumen. 9) Install 5” Galvanized Box Gutter and 3x4 Downspout to replace existing. 10) Pick up and haul away all roof debris generated by Presidential.

@Babatunde Onadele JrI would guess the winters and the amount of snow would have an impact.The lake is not salt water so you do not have the corrosion you have near oceans

Copper can not react well to the corrosiveness of concrete.