When I close the lid on my ThinkPad T420s, the half moon on the top lid turns on, notifying that the computer entered the sleep mode, but after a few moments the half moon turns off and the battery icon turns on and the fan starts up. If transported in this condition it seems to get warm.
Some common causes for this can be connection of USB attached devices like a wireless mouse or keyboard. If there are no attached devices, reviewing the file generated by the follwoing proceedure may help to identify which driver or event is triggering the system to wake from suspend / sleep mode.
1) Start an elevated (run as administrator) command prompt.
2) Run powercfg /energy. This will generate a report in C:\Windows\system32\energy-report.html.