A while ago, I installed an app on my website that displays Google reviews in bottom left corner. I assume the app has been deleted, but not the widget itself. I dont rembember the name of the app.
As Naeem already pointed out, some apps do keep their code in the theme file even after installation. And yes, you’ll need to remove it in the theme’s code.
I’ve done some inspection on your store and found where the code possibly is. It is inside theme.liquid. Not sure what it looks like in that file, but it is in that file and removing it will solve your problem.