This topic has gotten lots of traffic on many many threads over the years, on many forums. Ie.: curb strips, parking strips, or whatever you want to call them. Do a key word search, and you'll find very detailed pros & con's of both the legal aspects ("access" versus "public" versus "private" versus "eavesment", blah blah blah). And down to the ... uh ... practical applications of those legal things. Eg.: just because, yes you can walk there, stand there, etc.... can you detect there? And then it usually devolves from there, into a debate on if or how detecting is somehow innocuous (harmless) enough that it falls under the "normal usage/access" of such land.
And as can be expected, it divides down the line of the skittish types (who also probably think you need permission for each sandbox you come to too), will say you can't do it, you need to ask the city or the homeowner, etc.... To the less skittish like yourself, who just go.
All I can say is, while you may be right that you "don't have to leave", you STILL have to take into account "picking your battles". Because let's be perfectly honest here: While YOU may know you'll leave no trace, and while YOU may know the legal ramifications, yet, think about the connotations, to the person who lives there? a) he might think you're about to leave holes (even though it's not true, yet, put yourself in his shoes). and b) he maintains/mows it (again, maybe "so what" from a legal perspective, but again, put yourself in his shoes).
So while I would explain that it's public eavesment, yet I would not go so far as to have any sort of "in your face" attitude, and stand my ground.
Pick low traffic times, avoid lookie-lous, and .... give lip service and move on, if someone's not buying your explanations. The LAST thing you want, is for some sort of city involvement (if they were to complain), where you might get a pencil pusher to decide against you.