Infiltration will still work. Physical beings can still use infiltration against astral observers, it's just that the deck is totally stacked in favor of the astral observer.
First of all, the person on the physical needs to be sneaking. Maybe they had reason to be sneaking before they entered, perhaps not, but they won't have a chance to see the watcher spirit and know they need to START sneaking.
Second, there are tons of dice advantages the watcher will have. First there are tons of advantages just for astral vision, contrast between astral shadows and auras gives an advantage (even better for Astral Forms against a shadow background). Given that it's an office room, it will be astral shadows for a background, the players will have auras... good contrast. Looking at SM p114 looks like +1 (Background Illumination) +1 (Sparse Aura Noise) +2 Aura Contrast +1 (Inverse of BC, cities usually have a standard -1BC). We're probably talking a +5 dice pool modifier to notice an aura in the room, a +7 seems good to see an astral form, but we might take off a point for -1 Shadow Clutter depending on how much stuff is in the office. We're not even done with modifiers.
Up next we have regular perception modifiers off SR4A p137. +3 Actively looking, -2 distracted, either one could apply, or neither could apply. If the watcher is taking guarding somewhat serious then it shouldn't get the -2 distracted, but that doesn't mean it automatically gets +3 actively looking. Also there's a +2 bonus for object standing out in some manner.
And finally there is a "superior positioning" bonus as I like to call it. If the observer has an advantage to see/hear/percieve a certain location and the targets are in that spot, they should get a bonus. It can range from +1 to pretty much anything that is appropriate. This becomes a major thing with astral since the target will never know if they walk into that that area or not since they can't tell they're being observed.
All together, our watcher spirit that normally have 2 dice for Astral Perception would get something like 7 to 10, perhaps higher with positioning, when you include all the bonuses. And that's just a watcher, you can quickly see higher force spirits will be hard to sneak by.
And of course, if they don't know to remain out of sight then they should be considered "Immediately Noticeable" and no roll should even be bothered with.
Anyhow, I think the mage that made the watcher would be equally, if not more concerned with magical things in the office, so there would likely be orders for astral forms in there too.
