The point of a warrant is the independent audit. As long as that's enforced, it being at least partially in retrospect shouldn't be a problem; the issue being

The point of a warrant is the independent audit. As long as that's enforced, it being at least partially in retrospect shouldn't be a problem; the issue being how you would give that teeth in a way that's both not absurdly disproportionate yet still sufficiently discouraging. In any case, you'd at a bare minimum want a one sided notification a priori. I'd be more worried about whether judges really are independent and trying to critically reject warrants, rather than exactly when they do so. And another weakness in our current system: warrants are a pretty voluntary form of oversight. There are no reliable checks against warrant-less actions that should have required a warrant. Unless it's really egregious, police will not be held liable, defense attorneys may not be informed, and they may even be able to use the ill-gotten information for personal/legal gain, because finding a parallel construction once you know what you're looking for is obviously much, much easier. So I'd say that the problems are more one of a too-weak audit, and the moment at which that happens is not critical, even if it is relevant.