Unwelcome Flying Object can be found on the tower further away from the entry of Area 4. There are two ways of getting to Unwelcome Flying Object:
Option 1: Head to the tower on the right, there should be a geyser underneath the tower and an Inkrail when taken the Gusher. Activate it and fall down the hole. Agent 3 will be placed to where Tumbling Splatforms is located, and there is a Propeller-Lift to take. Use the Propeller-Lift and it'll take Agent 3 to a long cube platform with an Inkrail installed. Use the Inkrail and it'll take Agent 3 above Unwelcome Flying Object. Clip through the grate in squid form and Agent 3 will find the kettle to Unwelcome Flying Object.
Option 2: There is a shortcut to take underneath Area 4, when close to Propeller-Lift Fortress, Perform a swimming squid jump to the long cube platform. Use the Inkrail on the cube and it'll place Agent 3 above Unwelcome Flying Object. Slip through the grate in squid form and Agent 3 will find Unwelcome Flying Object.
Starting off the mission, Agent 3 is put in Urchin Underpass. There are filled crates, one of the crates holding armor. There is also an Octocopter hiding behind the crates. Not jumping off, there are Octotroopers to the left and right. Other routes are blocked off, so just continue on the mission.
Off Course: If Agent 3 goes into the alley of the map, there are 2 large empty crates. Breaking them will reveal two Octoballs. There are also two small crates that hold a lot of Power Eggs.
On Course: An Octocopter is guarding the entry to the central part of the stage, but that's not it. There are three Octotroopers and two more Octocopters guarding the other side. There is an Inkzooka there to fight them off. After they are taken care of, head to the next checkpoint.
There are now two Octotroopers guarding Agent 3's way through the grate, but there is a sponge that can be used for a shortcut or for hiding from the Octotroopers, but hiding from them won't be the exact answer since there is an Octostriker watching your every move. As you get past the two Octotroopers, now getting up to continue the mission, there are two more Octotroopers to fight. Once they've been taken care of, there is another sponge to use. A Bomb Rush Special is at the height of the sponge, which the Bomb Rush using Burst Bombs. Agent 3 will also face off two Octobombers, using the Bomb Rush will make it a small fight easy. Now onto the UFO Hideout!
Agent 3 is now against the Octostriker. The Octostriker hires Octotroopers to try to keep Agent 3 away. There is an Inkrail installed if Agent 3 doesn't want to go up above the small wall where the Octotroopers spawn. There is a way to make this fight extremely easy. If Agent 3 saves the Inkzooka from Checkpoint 2, The Inkzooka will do a ton of damage to the Octostriker. The same goes with the Bomb Rush special, but make sure it always hits the Octostriker. After the Octostriker's defeat, the Zapfish is in Agent 3's hands.