Just as the game suggest, it would be good to turn on Detective Mode, as it will make searching for the Guardian easier. The second one can be found south of your current position #1 #2. Just like before, you have to approach it and hold down LB to download further data.
Head even further south and find the Mechanical Guardian beside the small exhibition #1 #2.
Downloading data from the third Guardian will cause a new group of assassins to appear #1. Remember about evading often and you shouldn't have any problems winning. Return to where the third Guardian was and examine the opposite display #2 to collect further data.
In order to reach the fifth guardian, head to the north-west part of the Wonder Avenue and look for an upper ledge which you can access using the Grapnel Gun #1 #2.
After reaching the room above, look for another Mechanical Guardian #1 #2 resume downloading data in the well-known way.
Stay in the west part of the Wonder Avenue and look for another upper ledge #1 #2. Just like before, use the Grapnel Gun to reach it and stay cautious, as a new group of assassins will attack you. As the room is rather small, you might want to return to the lower level and continue fighting there.
Search the room in which the assassins attacked you and you will come across the sixth Guardian #1 #2, leaving only one machine left to find.
In order to reach the seventh Guardian, you have to go to the central part of Wonder Avenue. Look up and note that you can use the Grapnel Gun on the roof of one of the eastern buildings #1 #2.
After getting upstairs you should at once note the last Mechanical Guardian #1 #2. Approach him and finish downloading the video data. The game will play the material, thanks to which you will learn where to search for the secret passage.