I don't think familiars are meant to be fog-of-war removal machines. As A DM I would simply arrange some encounters with creatures that can see/sense invisibility (of which there are a few - also invisible does not mean you can't be detected) - it should be hazardous to send your familar far away like this, and if they die repeatedly due to your negligence/abuse, your patron should disallow you a new one: they are not supposed to be mere canon fodder.
