Is there any thing in the rules about mages constantly recasting Invisibility to get a large number of hits to ensure they won't be seen?
Not sure there needs to be. Cameras/Sensors get 9 dice to resist illusions, and drones 15. As a player you know you can't settle for letting 3 hits ruin your spell, or 5 hits if you know/suspect patrolling drones are in play. The GM *could* roll the dice for the player in secret I suppose, but it'd just do two things. 1) annoy the players, and 2) prompt the magician to start assensing his own spells to see how many hits he got and then deciding to recast anyway if he doesn't like the answer.
There's plenty of mechanics already in place to address permanent spells. Background counts, Wards, virtually any private enterprise wealthy enough to afford magical security demanding their patrons drop all active magic... You don't expect the team sammy to tote an assault rifle downtown do you? You shouldn't be expecting to walk around with a high force magic spell, either.
Also
I see Ultrasound devices have a rating from 1-3 and that the effect defeats visual invisibility. So the rating isn't relevant to see the invisible target just for, I imagine perception tests or hacking attempts use this rating. Otherwise the user can just the hidden mage.
The rating for the Ultrasound works like a perception test bonus. If you're "in plain view" no perception test is needed. If you're attempting to sneak amongst suitable camouflage, the Ultrasound works like any other sensor.
If the mage used Invisibility to mask sound then I think the silent area could be seen and then a perception test might be needed and would impose some sort of cover or blind fire mod to the shooter
Silence would end up working just like invisibility: Object Resistance test for the Ultrasound sensor.