I might be able to help you here...May I offer an alternative thought process to alleviate your irrational fear? Heres what I do!
Just dont bother door knocking at all! Just go and hunt anybodies yard you feel like without asking! Ya see, In the USA, the State owns all property even if its 'paid for' and the current inhabitant is just that, a squatter!...No different than a dirty hobo living along side the RR tracks! They never really own it, they just happen to be there at the moment...they can stay there as long as the property taxes are paid is all, so why ask permission from a dirty hobo or a 'renter' ?
Then, put in your mind you are a good guy who doesnt like to bother people...Nobody who is home wants a stranger to be banging on their door! You are doing them a favor by NOT asking! Just go give it a quick sweep to see if its even worth your while in the first place!
If somebody comes out and asks what the Hell do you think you are doing? Tell them in a stern voice to mind their own business! Tell them you are from the Gas Co and sniffing for leaks! Tell them you are from the local DNR Fire Ant brigade and digging around setting ant traps! Tell them anything you want or nothing at all! Make something up! Heres what I say....
"Ma'am, my instrument here is showing you have some pretty high levels of toxic Tri-Chlor-Ethelene back here underneath this old clothes line area, plus some trace amounts of heavy metals deposited pre-dating 1964 underneath this tree!" "Are you aware you may be eligible for some Federal Funds for the clean up of this possible Superfund site?" "You may also qualify to be the recipient of some Erin Brockovich kind of 6 figure Hazmat settlement!"..."Just let me do my job Ma'am, go back inside please while I get some samples, I will also have to take a bag of these tomatoes from your garden to run through the lab...Why Yes, I would like a ham sandwich and a glass of lemonade and maybe take a shower when I'm done if that would be no problem?"
"Thank you for asking...by the way, where is your Husband?...Oh, thats too bad, you probably have a whole lot of things around here I might be able to help you with...I'm from the Govt and I'm here to help!"....