My players are insisting that in order to see someone who's invisible, you have to know that you're looking for someone invisible. I don't like the fact that being invisible means you can walk right past someone's face and they don't even get to roll. But who does the RAW favor?

RAW doesn't favor them. To see somebody who is invisible, you just have to resist the illusion spell they are using. Same as to notice that the dragon eating the mall across the street is just a trid phantasm.

Your players are wrong, as an illusion spell anyone who sees it gets to make a willpower roll against the spell's net hits to see through it.

you can still hear and smell them

Ultrasound Sensor, no more invisibility..

I'd imagine cybereyes with thermographic or ultrasonic mods would take care of this.

Also the radar eye mod, enhanced sense of smell and hearing. Hell, if they are waving in your face you could even feel the passage of wind. Invisibility is only good for visible light spectrum so unfortunately thermo can't detect it (pg209 SR4a) but ultrasound and radar should be able to. Also, actively looking for something gives a +3 DP mod (pg 136 SR4a) so maybe they don't want you pulling that out on them.

Your players are wrong. People ”looking” for suspicious activities can use ALL their senses and I use the term “looking” for more than eyes, in this case one could perhaps say perceive for suspicious activities.

It can be the sound of feet on tarmac or the sound of movement through dry thick grass.

It can be how rain seems to have an “emty” spot that moves (as the rain will not be visible on an invisible person).

It is not about seeing an invisible person, it is about detecting that something is missing amongst sounds and effects that should have a source.

Things moving that shouldn’t move (invisible characters bumping into something, opening windows and opening doors.

Just because their characters are invisible does not mean that they don’t leave a trace and that their passing through an area is unnoticeable.

Ok, the target fails the resistance test and the team is invisible. The ground is wet, there are puddles all over and the ground is mainly gravel.

The hinges on the back door of the building is rusty and tend to creek when opened and one of the windows have a broken latch that can fall off unless one is careful.

The above are just a few factors that will influence the stealth and perception tests of infitrator and guard. Sure, the guard cannot see the target but might get the idea that something is wrong due to mysterious sounds and slightly moved objects.

Keep in mind that there are two different Invisibility spells and that can determine who and what is effected.

The first is simply "Invisibility", and is a mana illusion (resisted by Willpower.) It's pretty much the "clouding mens' minds" of the Shadow (or Obfuscate for you VtM players out there.) No amount of technological vision enhancement (even ultrasound) is going to aid in piercing this spell as long as the viewer is actually in the presence of the subject of the Invisibility spell.

"Improved Invisibility" is a Physical Illusion, and is thus resisted by Intuition. As the spell description states, as it actually bends light around the subject, it is capable of fooling technological sensors (cameras, drones, etc.) as well as those viewing through them remotely, ultrasound being the exception. To defeat ultrasound and motion detectors, use a Silence spell.

People automatically resist any spell they don't intentionally choose not to resist. (The exception is when they are unable to resist, because they're bleeding out or asleep or something.)

+3 on Perception for "Actively Looking".

I would think that a Silence spell would cause the ultrasound sensor to show some funny readings, though. Instead of it detecting the walls, or incidental things around, it would detect *nothing*, due to the silence effect. So that may at least make the guards curious (or they may just think the sensor's not working...until they test it in a different direction). Maybe a logic or intuition test for the guard to figure it out.

And for rules lawyers we could rename this to "Actively Perceiving" so that we all agree on that this is inr egards to ALL senses.

I thought about that. Someone with a Stealth spell on would show up clear as day on an -active- sonar, and they would show a kind of "shadow" on the passive sonar as the spell blocks the sound from other sound sources that reflects off walls. Much less obvious than letting the sonar pick up your footsteps or heartbeat, but it would still be possible to find you.

Spirits and astral perception will still pick you out as well. Would be really interesting to see someone using Invisibility from the viewpoint of a critter that's dual natured.

From an intuitive standpoint, I agree that a Silence spell would give ultrasound/sonar a funny reading, like a "shadow" or "hole". However, the description of the Silence spell states that it jams "...alarms, detection devices, sonar, and tactical communications..." I suppose you could read "jamming" a detection device as creating a "shadow", or you could just chalk it up to "it's magic."

Similar to trying to use thermographic or ultrasound goggles/cybereyes, I would think that a dual-natured creature would be fooled by the Mana version of the spell, but the book is against me, there. It does say you can cast illusions on the Astral Plane itself, however. So, if you don't mind sustaining two Invisibility spells at once (one Astral, one meat), you could certainly fool security critters. Or does the fact that all spells have auras blow your cover?

I would say they are both right and wrong. If the spell worked on the target he/she does not get a perception check to see them. They would have to actively look in order to find traces of them or switch to a viewing mode that will see them in the case of improved invisibility. They can still be heard and smelled but to use those senses in order to target them would normally require an active perception check.

There are a lot of cases where this will be automatic. Lets say I cast improved invisibility and get 5 successes, unless you have an intuition of 5+ there really isn't even a roll you don't see me and you have to figure out I am there from other things and then try to track me down.

But both spells are resisted spells so if they thought 1 success means there are invisible they are wrong. 1 net success does though.

Can't see you... but can see spell...

simple solution, hire ghoul guards

Another simple solution is simply have the guards have a few German Shepards with 'em. Dogs are cheap, loyal, and above all, SMELL things.