Although your characters probably don't know this, summoned demons need something to tie them to the material world, or they end up drifting back into the warp before long.  If nobody looks at a demon, they cease to exist.  Now of course, the sorcerer is something the daemons can use to anchor themselves to reality, but that's why he's the center of the incursion, rather than the location the daemons were summoned in.<br/><br/>Naturally, it's very much an in-character choice to decide if you should wait for the fighting to die down, and a successful scholastic lore (occult) or scholastic lore (Imperial Creed) roll would reveal the information about daemons vanishing if they don't get enough attention.