I use my MacBook Pro at home plugged into the mains supply. The battery appears to be fully charged. If I turn off the computer, a few hours later the connector light is orange and the time to recharge shows about 15 to 20 minutes. What is going on? 
Eric Matthews

There are two possible answers here. If your machine is sleeping rather than turned off, it’s possible that your Mac is running background tasks while you’re gadding about. So, in short, yes, your machine may be turning itself on during your absence – if you’re leaving it in sleep. To confirm this, immediately upon restarting your machine open /Applications/Utilities/Console and go to File > Open Console Log. Look for signs of activity during sleep and take steps to disable them. A good first step would be to switch off AirPort and Bluetooth before bedding down your MacBook for a time. And, of course, make sure you shut the machine down from the Apple menu.

The second answer is that, if you are actually turning your machine off, it could be a battery issue. The clue is here: “I use my MacBook Pro at home plugged into the mains supply.” If you do that consistently, it will shorten the life of the battery. It’s possible that the 15 to 20 minutes recharge time reported the actual capacity of your battery. You should try to run your MacBook on battery power only, at least twice a week, right down to automatic shutdown. Then give it a full charge from the mains.