Why is my phone overheating without using?

Hello,

I am writing to ask about my phone. I have Samsung S8 since December 2017. About a month after I'd got this phone, a problem occurred. It was overheating when I was talking on the phone. A few minutes were enough to make the phone heated to 40 Celsius degrees. It was quite so high temperature to keep the phone in hands. I've visited Polish flagship service called Samsung Service Plaza in Warsaw for 7 times. Every time I left my phone there and got an answer that 'everything with the phone is ok' and it's defect free.

But still, the problem occurred. Then my phone started to overheat after having mobile data connection turned on for few minutes. Even though I didn't use any apps, the phone had a high temp.

Last week I had another accident with my phone. I had it tucked into my pocket, with Wi-Fi, data connection, BT, localization or anything turned off. When I put my hand there I felt a heat. The phone was overheated again, this time without using it.

I compared the phone with few other S8 devices. There wasn't problem like that in any device. But flagship service of Samsung can't do anything and don't even see any problem.

When I called Samsung Guard helpline I've heard that there is no problem with overheating in my device because it would be possible to name that 'overheating' only when it reach over 100 Celsius degrees.

I don't know what should I do with that fact. Did you have similar problem? What could possible heat the phone like that?

Greetings