I had a similar problem: a 3rd party home button on my iPhone and the battery emptying in 8hrs on standby.
The repairman took the screen off and the A9 (black square towards the top of the phone) was too hot to touch, even on standby.
What fixed it was to get a new 3rd party home button from a "different factory".
Right now the phone has been on standby for 16 hours and the battery is still at around 90%.