I noticed a couple of days ago, my bathroom floor (right next to the toilet) and the hallway floor is warm. Why is this?

Are there heating ducts running under the floor in those areas? If so, and if they are not well insulated, heat could rise through the floor...

This is a goo9d indication that you might have a hot water leak under the floor in that area. I am assuming that your floor is a concrete slab floor, but this could still apply if the floor is wood and the water is spraying against the floor.

Go to the water heater and use a long screw driver, place the tip against the hot water pipe coming out of the heater, place the handle to your ear and listen for a running water noise.
If you hear something you can now turn the water off at the inlet valve and listen again at the hot water pipe. If you hear a sound with the water on and do not hear noise with the water off that is a good sign that you have a leak.

If you decide that you have a leak let us know and we will tell you what to do from here.

Let metellu gave you one method tocheck for a hidden leak. Let me give you another.
To check to see if you have a hidden leak first shut down everything in the house, no flush, no drinks of water, and if you are on a meter go out and check the little pointer in the gage. It should not move or creep. If it does you have a leak. If you're on a pump then check the pressure gage after the pump builds up to pressure and shuts off. The gage should not fall and the pump come back on. If so then you have a leak. If you find that you , indeed , have a hidden leak the next thing to do is pinpoint it. Water, like electricity ,will seak the easiest path so the actual leak may be feet away from the hot spot. We call a company, such as Sleuth, that locates the leak, jackhammers up the slab, bags the dirt and exposes the leak so the plumber can repair it. Another method would be to locate the pipe in the manifold in the wall and cut the old line out and loop a new one up in the attic. Either way something's going to get tore up. Let us know ehat you decide, good luck, Tom

It may be under slab leak...

1. Does the floor feel cold first thing in the morning BEFORE you run ANY hot water in your house ?
2. Do you have hot water circulating pump installed on your plumbing system ?

Please, provide this information so we can better identify your plumbing problem and offer appropriate solution. Milo

So now Smile, all you have to do is tell us if you have a slab under the floor becausemost of us are thinking burst hot water pipe. Did you try Milo and Tom's suggestions and what did you find?