[tonyg]

There are a couple of approaches here. If there is a moment when it's known that no matches will be found, then either an actor can assert something to this effect or the querier can wait "long enough" and then move on. For the former, if there's some actor in charge of maintaining an authoritative copy of the collection concerned, they may assert some "no match" record. For the latter, the SAM includes a "sync" action which is essentially a no-op roundtrip to some target. Once the sync returns, you know all previous assertions must have been seen by that target, and if you know something about how the target operates, it might be safe to conclude no response will be forthcoming.