How to unroot automatically rooted mobile?
1- Yes, some malicious Apps (even on google play) may use exploits to root your device and gain super privileges.

2-If you want to check root, you may use apps like root checker as you did or you can search for su binaries in system paths.

3-If you want to fully unroot your device my advice is to use supersu app and let it unroot your device or you may want to flash your device with stock ROM.

But flashing even with stock ROM may result in voiding warranty if you are not familiar with the process.

To be in the safe side, If you're sure that your device is rooted, you can simply unroot it with SuperSU application and check to see if the device is completely unrooted.