There are a lot of secrets, yeah. Here's what I know:
The gladiator stage has some wall buttons (and the center of a tapestry) that you can shoot to drop down the nearby ceiling and instantly kill anyone underneath. It's great to lose a pursuer after crossing through the bounce pads. There are also small medusa faces at the top of two or so archways. If you shoot those, the face will shoot a wide beam that turns anyone who crosses it to stone for a few seconds (they have to break out). They're a little trickier to use, but can also be great for losing pursuers, or trapping an opponent for tasty damage. You can also blow off pieces of the dead cyclops to move through it a little easier. Not sure if there's more.
The Alientown map has a lot of secrets. There's an alien tentacle wielding a car in the top right corner of the map (from spawn fly-in). If you shoot it, the tentacle will swing three times, and instantly kill anyone it comes into contact with. I narrowly won a deathmatch game using that. There are are a lot of hidden areas and warps too. There's a plain drawing warp on a wall in the bottom right corner of the map that will take you to the top of the buildings in that area (there's a grannies pies billboard up there that you can shoot to get a trophy. Lots of map aesthetics change when you shoot them.). You can jump into the fountain in the center of the map and there's a little sewer area with a few weapons, and a health pickup next to a warp that takes you to the center of the building on the left side of the map. In the upper floor of the north end of that same building, there's a bathroom door that you can shoot open. Again, there's a health pickup and a warp that takes you across the map. I don't think I've gotten the trophy for using all the warps in one game yet, so there's probably more stuff I don't know about.
In the bloody mansion map or whatever it's called, there's a large portrait of the dissected frog in the upper area of the map that--when approached--opens up to reveal a long hallway with a health pickup at the end. There's also a ghost holding a candelabra at the end as well. If you shoot it, the ghost will quickly charge through the hall and instakill anyone in its path. There's a warp at the bottom of the map that takes you back to the top, but it's not particularly hidden. I don't know of any other secrets on that map.
On the Flooded map, there's a lady liberty statue wielding a gun with a health pickup on the end. If someone is going for the pickup, you can shoot the statue's gun and it will in turn shoot and kill whoever is standing on the barrel. I haven't explored this or the other maps too thoroughly yet, so I don't know of any other secrets.
For general tips... I like to plant mines on the outside of warp gates, on the way to health pickups, and near bounce pads. Stuff people use in desperate situations, or stuff you use while running away. I get some nice kills that way. I also fly around a lot and use the RPG and Patriot Act like a jerk. It's worked out pretty well so far.
