Great idea! I am going to get some little cones and try this when I head out this weekend.
Side note: If you knock on a door and get permission what do you do when a person asks to see what you found? What if they want it?
You definitely want to check out the historical aerial photos to identify any old structures. Houses, gazebos, ball fields, etc...
As to your question about ownership of finds, once you've received a few private permissions you're basically the expert in the subsequent interactions with property owners. You'll find some property owners who understand what you're asking, but most only have the vaguest idea of what you're asking. You can either use that experience to make self-serving assumptions to take advantage of them, or use it to make sure everybody is on the same page and happy.
I've worked out lots of arrangements. Everything from "I get to hunt and keep whatever I want without showing anything" to "I'll show you everything I find and let you pick whatever you want to keep." Whatever the deal, I stick to it.
A property owner is making things easy for you if THEY bring up the finds ownership issue before the hunt has even started. Don't like their terms? Negotiate, or don't hunt. Agreeing to their terms and then hiding your finds is trashy at best and theft at worst.
Metal detecting is much more fun when I don't feel like I am looking over my shoulder trying to hide something. I also hope I can find something I think the owner would like to have, even if they don't ask. I already know if I ever found the proverbial jar of gold coins or a significant historical artifact, then there's no way I would hide it from the property owner. I'll hope the person is willing to throw a few of the coins my way. So, for everything else, none of it is more valuable than my reputation or peace of mind.
But, what about the gray areas? What if nobody clarifies the issue of finds ownership before the hunt starts? That's what a lot of people seem to do. That's fine if you want to take that risk, but take your lumps if it turns out the owner does want to see the finds and possibly have something before you leave. No need to be a liar just because you took the chance that they wouldn't ask and this time it didn't work out. I'm sure there will be other times when you'll incorrectly assume the person wasn't interested and leave.
Some property owners won't ask about the finds because they truly don't care, and some can't imagine you'd ever find anything they'd be interested in seeing or owning. (We love these people!) But, other owners might make the reasonable assumption that you'll check in before you leave. Imagine their surprise when they look outside to check on your progress and you're long gone. (I've had permissions denied because they said a previous person did it to them.)
That detectorist poorly represented the hobby AND missed out on potential leads to other properties. Suprise the owner with your honesty and skills and the next thing you know you have permission to dig on their family farm or a historic property they just purchased to renovate. It's happened to me more than once.
An indirect way of clarifying the finds issue up front is to simply ask "Would you like for me to knock before I leave?" If they say "Don't worry about it." then that's all you need to know. Go have fun. If they say "Yes" then you can either say "OK" and continue to take your chances, or go ahead clarify the finds issue.
What I've been talking about is door knocking when the property owner is home. If I'm still detecting and they leave without saying anything about following up, then that's a pretty good indication they didn't expect to see the finds! Similarly, if it's a property where they don't live, such as a rental property, vacant lot, or abandoned home, then I think the responsibility is theirs to "speak now or forever hold your peace" when they grant you permission. Still, I have often followed back up with those owners with photos of finds, including things I'd prefer to keep and nothing has ever come up where I had to give up something I wanted. I'll even send them one of the older coins or a relic I've researched.
So, what happens when a property owner actually does want something you'd like to keep. If and when this ever happens to me, I'm just going to tell them I'd like to have it for sentimental reasons and not because I'm going to sell it. If that's not good enough, I'll give them my card and tell them I'll follow up in a week after they've had a chance to research the item or run the decision by a family member. Hopefully they'll have a chance to be satisfied I wasn't pulling one over on them and they give it to me for free or offer a fair price for it.
Also, don't forget to get photographs before you hand anything to a property owner.