Many laptops now come with Bluetooth*.
Just like WiFi, Bluetooth has a radio transmitter, so it can take quite a bit of power.
Unlike WiFi, Bluetooth is rarely used, so it's very possible that your Bluetooth device isn't actually used for anything, and is just consuming battery life.
You can see if there are Bluetooth devices active on your system with the hcitool command:
hci0: Type: USB
BD Address: 00:00:00:00:00:00 ACL MTU: 0:0 SCO MTU: 0:0
RX bytes:0 acl:0 sco:0 events:0 errors:0
TX bytes:0 acl:0 sco:0 commands:0 errors:0
In the example above, a USB bluetooth dongle is present and active.
Note that many internal bluetooth adapters in laptops now use USB.
The easiest way to disable a USB bluetooth device, is to use these commands:
hciconfig hci0 down