I had an unusual situation a few years ago. I arrived at a customers farm, the shed doors were wide open so I get situated and get started on the task I was hired to do, A/C on a 7140 IIRC. (he knew I was on the way)

The customer comes out and rips me a good one for entering his barn without him being present.

The odd thing is, we've known each other for a long time and his cousin works at the same dealership that I do. I used to do a lot of service work for his Dad (going back to the mid '70s) who had no problem with me working at his place unsupervised. In fact, our Dads were high school classmates.

I wouldn't ever help myself to a customers equipment unless it was one that I knew wouldn't mind if I did. I would also be apprehensive about using something I didn't know how to operate.