rMBP put to sleep, charging. came back 10 hours later & found this...

[VideoNewbie]

i came back and found the bottom of my computer was still slightly warm, why?

i dont ever recall it being warm when its not in use. even while charging

 

[mobilehaathi]

I wouldn't worry about it.

 

[T5BRICK]

i came back and found the bottom of my computer was still slightly warm, why?

i dont ever recall it being warm when its not in use. even while charging

http://support.apple.com/kb/ht5394

Additional Information
When Power Nap is enabled, you computer may be warm to the touch even if it is sleeping. Power Nap operations will be suspended if your computer exceeds a predefined temperature; your computer will not overheat when using Power Nap.

Or it could just be that it was charging. That can cause some heat.