Hello. This guide will show you how to find each hidden SMG and the single hidden katana in Dead Rising as well as the Deluxe Remaster. These are very good early game weapons, that can take you into the late game quite easily.
Press Select on your controller/M on the keyboard to open the map.

The first hidden SMG can be found just outside the Warehouse inside Paradise Plaza.
First, on the right set of stairs facing the balcony, jump above the railing to the blue platform above like so.
Continue along the blue platform, passing the orange juice and teddy bear. At the very last blue platform will be the hidden SMG for Paradise Plaza.

While you are here, grab the hidden katana if you'd like.
Right off the balcony of Colombian Roast Masters on an orange awning is the katana.
Drop down from Colombian Roast Masters off the railing closest to the Cafe.

The second area you should encounter, Leisure Park's SMG is tricky to spot, especially at night when fighting the convicts.
To the direct right of the Parking Lot, there's a collection of trees.
Up in the biggest tree on the branch furthest from the brick wall is the SMG hiding on the branch. You will need to throw something to get it to fall down.

During or after your fight with Carlito, the food court has one of the easier SMGs to reach.
Right as you enter and to the left is the Chris' Fine Foods mascot.
Jumping onto the golden railing, and then up onto the base of the mascot, the SMG will be on his right side.

To the left of the Food Court, where you followed Brad, is the Al Fresca Plaza.
This SMG location is a bit dangerous as Al Fresca Plaza is usually jam packed with zombies.
All the way down in the direction of the Entrance Plaza, there is a big fountain.
Inside the fountain is the SMG, submerged in water.

This one is thankfully much easier to reach in the Deluxe Remaster. All the way at the top of the escalators stand in front of Estelle's Fine Cosmetics.
The second palm tree to the left facing away from Estelle's Cosmetics has the SMG floating above it.
Just like Leisure Park you have to throw something at it to knock it down.

The SMG in Wonderland Plaza is annoying to reach to say the least. It's the only one that has a timer attached to it, as you have to ride the space ride to get to it.
On the left side of the space ride relative to Adam's chainsaw, is a yellow LEGO-ish house with a red roof. There's one on the other side as well so remember it's the one closer to the North Plaza.
From 10:00am to 10:00pm the space ride is active and circles the Wonderland Plaza. Stand where Adam croaked and wait to step into the ride.
Almost immediately after the ride starts moving, just a little before you're above the LEGO-ish house, jump off the ride and try to land on the red roof. On top is the SMG for Wonderland Plaza.

All the way down to the Parking Lot and into the Maintenance Tunnels, take an immediate right then an immediate left, then it's on your right.
The location is crawling with zombies which makes getting to the door in the back much harder. I killed them for your viewing convenience. If you remember, this is the same location you find Zombie Brad after Carlito leaves him to be eaten.
In the Maintenance Room, to the right is the Maintenance Key, and to the left in the back behind some barrels is the SMG.