Matt Bird writes all sorts of nonsense, but he dedicates a large chunk of his time to writing game walkthroughs.
Are we alone in the universe? If you're talking about LEGO, nope, not a chance. Turns out the aliens are on the move, and it's up to Chase McCain to keep them under wraps by teleporting them in with space crates and slapping cuffs on them for a Gold Brick apiece. This article covers the locations of all 17 aliens in LEGO City Undercover. Be prepared for a short run and a takedown each time you teleport in an alien - they won't go down easily.
Cherry Tree Hills
Along the side of Cherry Tree Hills, near the Heritage Bridge, is a series of high-rise buildings. At the end farthest from the bridge is a small eating area with picnic tables. The space crate is sitting here.
Near the Auburn Bay Bridge is a large gas station called Hank's. There's a jump ramp here you can check under to find the space crate.
When you enter Fort Meadows you'll see a gas station on the left side of the road. On the right, across from this gas station, is a pile of shiny rocks hiding a cave. Blow them up with dynamite and you'll find the space crate hidden inside the cave.
Bluebell National Park
Near Bluebell Mine is a natural stone staircase you can jet up to find a few different collectibles, a space crate included.
After you've saved Natalia on the front half of Mercy Hearts Hospital, make a return trip and check out the back. The space crate is atop the rear entrance of the hospital.
Early in the game, during one of your first visits to Pagoda, you had to climb a building across from Chan's Scrap Yard and scan across the street for audio signals. On the way up to this point you passed a space crate atop an intersection of four buildings. This contains the alien.
Bright Lights Plaza
Check the outer walkways on the second story of the central mall (the one with the enormous disco dancer) for a way up to a teleporter. It will zip you into a glass enclosure with a space crate, among other things. Activate the space crate to beam in the alien outside then teleport back to your previous position to jump down and give chase.
There's a waterfall near the tea house in Crescent Park. Check behind it for the space crate.
Have a look around the stores of Uptown for a high-profile toy store. Across the street from this toy store is a slightly-less obvious bakery. The space crate is on its roof.
LEGO City Airport
There are a series of warehouses and sheds on the Airport's property. Break into one of them to find the space crate.
If you look along the eastern edge of Fresco you'll find an ATM at the edge of an alley. The building directly across this ATM can be climbed via poles and flagpoles and the usual assortment of blue and white bricks. Up here, among many other things, is the space crate for Fresco's alien.
Along the strip of hotels is one called the Herbert Hotel, the last one you were on before you managed to overhear a conversation between Vinnie Pappalardo and Rex Fury. The space crate is on this roof.
Down by the docks is a Ferry Super Build. Beside this Super Build is a small shed. Climb atop this shed to find the space crate.
Up the road from Downtown's border with Kings Court is a cluster of buildings near an upright statue of a ring. Atop the building with the red pot at the side is the space crate.
Not far from the gate to the rear of the prison (you can't actually go through it) you'll find the space crate atop a low roof.
Once you have the Jet Pack you can easily get atop the Space Centre to find a bunch of different challenges and collectibles. Among them is a space crate that will beam in an alien.
Lady Liberty Island
As with most things on this final of final islands, the last space crate is easy to find. Look for the Pig Cannon of Lady Liberty Island and you'll see the space crate sitting nearby.
What you get for snagging all seventeen aliens is exactly what you wanted all along: the Space Alien outfit. You'll also get a Gold Brick for your trouble, but, meh. That is nothing to the ability to run around as a mighty alien.