If , as you say, all sorts of city employee people, and passing cop have no problem or issue with your detecting , then .......
what more can you ask for ? Red carpets to be rolled out ?
To ask "does this constitute permission?" is just a semantics game/issue. No matter if you call it tacit approval, permission, or just "no one cares", then no matter how you slice it, I'd continue.
In fact, why does metal detecting "need permission" anyhow ?
No more so than you'd think you "need permission" to whistle dixie, or whatever. In-the-absence of a prohibition, then no permission is needed.