How are the players going to figure it out?This is basically the equivalent of you leaping up behind from your GM screen, and shouting "Spoilers!" Why not let the party figure out what the circle is first before you tell them what it is?
Either they ask the GM and the GM tells them (perhaps gating that telling behind something else, like making a certain roll on the dice).
Or they conjecture, and the GM confirms their conjecture.
More generally, @hawkeyefan is positing that what is interesting in play is not a relatively drawn out process of conjecture and confirmation that results in the GM telling the players that yes, indeed, it is a circle of protection. And rather is the players actually deciding what they want their PCs to do about the circle of protection (eg to use it; to lay a straw across it; to erase it; to determine its origin; etc).