Is it the plug itself that's heating up?

(this would be the sign of a bad connection)

Or the charger body itself.

(This would mean the the charger is overloaded somehow)

Or the battery?

(bad batt/ battery connection)

My guess there's something environmental involved since it's happening at home but not at the shop.

The techs had it set on a nice clean level bench with an outlet right close by when they tested it.

Do you have the charger plugged into a long cord? Are you in an older building?

(Low voltage can cause AC stuff to heat up. Long cords cause the voltage to drop, older buildings can have really nasty electrical systems.)