How to trace mystery switches
Guessing from experience, they are probably to outdoor lights that you don't use, to a switched outlet that was converted to be on always, or to a fan/light combo that is no longer wired to separate switches. 

The outdoor lighting, check for any lighting fixtures that you can't turn on, make sure you have a good bulb and start turning on switches.

The outlet option can be checked by removed the wall plate and checking for any extra wires (can typically be done without removing the outlet from the box, just use a flash light) evidence of an extra wire does not guarantee its use but absence of one means it's not that. The outlet option is typically the lowest chance if you have an overhead/ceiling light. If it is still wired in, you can go with the typical circuit breaker to outlet locator method and plug in a radio or other device to an outlet, that way you don't have to be in the same room area to detect power, just make sure powering on/plugging in actually makes noise, most digital models require you to push a button.

The fan/light scenario is most common in a bedroom where there are multiple switches in the same box, always possible in bathroom for previous vent/light combo. This would require actually checking above the fan which most of the time means removing the fan at least from ball joint.

An generally, finding out which circuit the switch is wired to may help located what it is to, although there is a chance it can confuse the situation even more.