I have a small farm (sheep, chicken & turkeys). I'll give you some tips at what upsets farmers.
Gates left open - this one is HUGE as we're responsible for what our livestock does, they're expensive, they're stupid, and they don't know cars hurt. It's also a pain to try and get them back into a pasture if you do find them.
Crop damage - holes don't matter so much as stepped on and knocked over crops.
Liability - I know if I give you permission and you step bad on a rock and break your leg or get tangled up in a conveyor I will end up liable.
So, when you come to ask I'm going to be a lot more favorable if you tell me ahead of time: " I'll make sure your gates are closed and even keep an eye out for loose fencing, stick to the rows and be careful near the crops, and even though I seem concentrated on the detector I'll pay attention to moving equipment and where I'm walking."
And it NEVER hurts to say: "I'll show you what I find and you can have all the personal items but I'd like to keep the coins."
And here's a tip. There are usually tons and tons of old iron around farms if they've been there very long. Unless something big is in the tillable parts it's probably best left buried. I'm more annoyed with a huge hole than I am happy to have you find a horse-drawn cultivator with 80% of the tines snapped off.