Facebook Pixel nowhere to be found in the code.. but still tracking. Where is it?

[RobinNoe]

Hi everyone, 

a customer of ours has a pixel installed to track activity in his shop. That's clearly indicated by the chrome extension "Pixel Helper", it's firing shots reliably.

Problem: it is nowhere in the code!

He doesn't know himself how it was installed. But I checked all liquids - it is not there. The Facebook App is also not connected/set up in his backend.

There is just no trace of it. He also doesn't use Google Tag Manager.

How is it possible? 

We want to make sure it's implemented correctly, but that's hard to check if i just can't find it.

Appreciate your help a lot!

Thanks & Best
Robin