Is that something which has changed more recently? (Since Sept '15?)
In my set up, Sonarr certainly did some searching of it’s own in a similar TVDB scenario, without any manual intervention/searching, back when they renumbered all the Mythbusters seasons.
I got the impression from that situation that the logic was something along the lines of “an episode has been added to a monitored/new season of a monitored show, and that episode aired in the past: run a search for it automatically” (something which I’d agree with, in theory).