need a plumbers advice


Addicted to salt
So a few weeks ago i installed a 20 micron whole house water filter inline with a Whirlpool 3 stage whole house filter as well as a new water heater. Here is my issue, I have the feeling I did not tap into the main house cold water line far back enough because today when I turned off the water at the water heater to replace a P-trap under the garage sink, I still got cold water. This instantly got me saying ***? is there another main gate valve i am missing? what should i be looking for from the outside? the house was built in 78 so it's mostly copper tubing and today the only thing i noticed was outside next to the electric meter was a small maybe 1/2" copper tubing coming out of the ground and seemed to enter the garage on the opposite wall of the water heater.

any help will be appreciated