You know those occupancy-sensing light switches you often see in commercial bathrooms and the like? The ones that turn on when they sense heat or motion and off after a few minutes?
Are there similar devices that simply beep an alarm to remind you to turn off the lights but aren’t actually in control of them?
Here’s a link to a motion-sensitive night light available at Home Depot:
http://www.homedepot.com/h_d1/N-5yc1v/R-202077647/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053
They actually have a number of motion-sensor lights available; you might want to take a look to see if any suit your needs.
Thank you, but I was wondering about beepers/alarms, not lights. I know the lights exist 
I think you’re going to have a hard time finding something like that (not to say it doesn’t exist) since if there’s no one in the room, there’s no one to remind to turn the light off, and if someone is in the room, they’d still want the light on.
Yeah, it is a little unclear how or at what point you wish to be reminded to turn off the lights. Lots of stores have motion sensors at the doors that will alert the staff with a beep that someone has entered the store. Is that the kind of thing you had in mind?
I thought of why you may want such a device, and came up with if you were blind but sighted people used the switch. This way the blind person could know if the switch was left on. So perhaps look for such a device for the blind.
You could get a beeper (noise maker) of some sort that goes on with the light.
Also why would you want such a switch? 
You might want to think twice before installing any of those devices. We just built a new town hall, equipped with all the latest technology – automatic flushing toilets, motion sensing lights, etc. and have found that they aren’t a panacea.
We have already replaced one room with a standard toggle switch for lights. We were unable to adjust the sensor to stay on when someone was using the room (it often shut off without warning) or to turn off when no one was there (it was often already on when we opened the room). Ostensibly, the intention was to save electrical costs, but not only was it a holy hassle, but it probably cost us money after all.
Also, when you walk into a dark bathroom, you have to hesitate before taking another step to give the sensor time to turn stuff on. Just a second or so, but it’s against human nature to walk into a room blindly and hope you will get some light before you crash into the wall or another person. So we either hesitate or yell something, like “geronimo!” before pushing the door.
prompt people to wiggle or do the wave every five minutes during meetings.
newer sensors have multiple sensors/methods in a unit and might work better.
training and behavior can be the best conservation method even when mostly or totally manual controls.
Well, that’s what I get for trying to read an OP before I’ve had my coffee! Sorry. 
If a motion sensor doesn’t detect a person in the room and beeps and no one is there to hear it, does it really make a sound?
Only if it’s in the forest.
Or something.
Wait. What? 
I put a motion sensing light switch in a public restroom. ‘Management’ was really only concerned about the light being on all night, so I set the on time for 20 minutes. I figure if you have not moved enough to reactivate the sensor in 20 minutes, maybe the electronics do need to ‘tweak’ you a bit.
I’ve been putting some X10 home automation in my house, and it could easily do what you want. A very basic explanation is that X10 is a protocol (and there are others) for sending signals through the home wiring to devices that switch things on and off. It is generally set up to be controlled by computer but can operate stand-alone as well and also be controlled with remote controls.
It would be overkill if you only wanted to do one chime for one room, but illustrates that it is possible. Off the top of my head, the controller I use is $40 but I write my own scripts, it could cost more for different controllers and fancy programs, a motion detector $10-15 and I think a chime is about $10.