Hi @MarcMuc, welcome to the Sonos Community!
I’m sorry to hear you’re having issues with your speaker volume changing without your input.
There aren’t many reasons that volume would change without your input, so if you can rule out any home automation your system is part of or any housemates that are messing with you using another controller, then a deeper look into your system would be needed. I would recommend submitting a diagnostics within ten minutes of the volume increasing by itself then contacting our support team and giving them the diagnostics reference number. They have the necessary tools available to look into your system and see where the volume commands are coming from.
I hope this helps!
Don’t overlook the silly stuff, such as a stray finger or thumb while you hold the pad/phone.
While I’ve never personally observed it, I can imagine a blob of jelly at the screen edge causing issues. If I’m using my iPad in bed, sometimes a corner of the blanket will interfere with touchscreen operation.