I've read about wards in both SR3 and MitS and still don't know whether they affect physical space. Could I put an alarm ward on a door that notifies me when someone opens it?

Maybe if you strapped a hellhound puppy to the door and set an alarm ward just within the door, so when you open the door, the dual-natured hellhound trips the alarm...

You normally can't have physical thing trip a ward, as wards are astral barriers. Alarm wards (or any other ward for that matter) are triggered by things that are active in the astral plane.

You could however stick a small sensor in a tiny phial containing FAB bacteria (which are dual-active). Have the sensor trip an alarm when the bacteria kark it.
The bacteria kark it when the phial is shoved through the ward across the door

[Edit] - this idea is based on what I've understood of the biotech they're hoping to use IRL for cheap and fast detection of biowarfare agents etc...

Or just use Leeches (from RAS, I believe). They do nasty squeeky noise-things when an astral form passes through them, even though they're not astrally active.
A ward might trigger that kind of response as well...

That's if you want to trigger a physical alarm system.
If you just want to set off the alarm ward, a small bunch of FAB should do the trick.
Or anything else that's dual-natured and door-attachable.

Don't mind that stuff under your foot. That's just my brains. They've been oozing all day.

Edit: Nonsense wonsense.

Actually Demo, the ward would let the caster know as soon as the bacteria pass the ward. If the goal is just to let the caster know, the physical sensor isn't necessary.

Putting the leech on the door (and they show up in MitS as well) would emit a noise on the physical plane I'd imagine (the leech reacting to the ward) but it might not alert the ward, since by the way MitS is written it's not clear whether the leech is dual-natured and thus present in the astral. Of course, I think this is an oversight, and ruling that the leech can sense the astral without being present would open up a huge can of worms.

It seems like it might be easiest to just whip up a watcher (for a relatively short term situation) or actually design a spell to do this. Although come to think of it the spell would have to be sustained, which means you'd have to keep LoS to the area (which kind of defeats the purpose).

(Note: I'm guessing that you have to maintain LoS to sustain a spell. I didn't find it in my quick look through the book)

Assuming you have an alarm ward up, you could just quicken a low level spell on the doorknob or something or tie a focus on it or something. Both of those things are present on the astral and so would set off the ward.

By MitS leeches are still a bit hard to come by, but there is BioFiber as well. It's a dual natured wood substitute. Just build the door out of it. This has the added advantage of no one wondering why the door has a vial of some kind attached to it. They just say "oh, biofiber, guess they didn't want anyone going in on the astral".

The Other DSE

Biofiber doesn't exactly grow on tre... erm, anyway, Biofiber isn't something that you can just carry around with you in a pinch, if you were holed up in a rented place (like a hotel room).

All beings can detect astral entities passing through them now. SR3 p 173:

Although there are several gimmicks you could try such as the ones folks have already suggested, the practical answer to the original question is no. There are easier ways to detect an intruder opening a door than an alarm ward/hell hound/leech construct combo.

I think I'll use the FAB + ward combo for one of my magic characters should I ever play him again.