There is usually a delay of some sort to the radar in a Scenario.
Typcially, what this does is simulate just the type of tower reporting that has been discussed here. Typically, no one wants that job, delaying radar simulates someone spotting a formation, calling it in, and having that information sent to the pilots in the air. A delay. An intentional "There is something over there, somewhere, go find it"
Scenarios most likely won't ever have a full blown Main Arena instant on type of radar element. That defeats the intention of a scenario.