100 Watts on for a month will use 72 kWh, which is $7 to $10 based on current rates. An attic fan or lights, or an outside circuit, or an electric heater, could use a lot of power. A humidifier could use a lot of power.
Another cause would be a pump that is operating continuously because of a leak or because of a pump failure that is preventing it from reaching the shutoff pressure. If you have a submersible pump in a well, that is the first place to check to verify that it is shutting off.
An air-conditioner with a heat pump could use a lot of power.
A water leak in a hot water pipe could use a lot of power if you have an electric water heater. If you are supplied through a water meter you can turn off all known uses and see if the water meter is still registering usage.
If you are comfortable working with electrical things, you can check it out as follows, using a clamp-on ammeter which you can get at HD.
1. Make a list of each circuit breaker in your panel, including what it serves.
2. Turn off everything you can in the house.
3. Take the cover off your main panel.
4. Use the clamp-on ammeter to measure the current at every circuit breaker. Also, measure the current in the main lines coming in if you can do it safely.
Now look at the current readings to see if there are any circuits that are using more current than you can account for where 1 Amp is about 120 Watts. If you think you have turned off everything on a circuit, and you are still measuring current, then there is something that is operating that you don't know about.
You should also check to make sure that a neighbor hasn't hooked something into your circuits. If you have turned everything off and can safely reach the incoming power lines you can measure the current coming to the meter. That would reveal if there is any connection before it gets to your main panel.