Testing, Testing ... I replied to this thread earlier, saying more or less this, but it doesn't seem to have appeared. The spell, "Watching Ward" is a ritual, yes, but the guideline doesn't say that that is required, and none of the other ReVi or MuVi metamagics require a ritual. As such, we've always concluded that the special, indefinite duration was what made the spell a ritual (and thus needed a labtext or experimentation to invent).
So, the duration of Watching Ward really is comparable to "Until", since it ways until something specific happens?
Since this does not include Year Duration (per se, although potentially forever), Boundary Target nor Permanency with Creo, it should be perfectly legal for a device. If this is written in HoH/MC-Verditius, I shall have to look it up. Not that I don't believe it - that is the logical place for it to be - but out of curiosity.
Serf's Parma (books home, not read Watching weard in a while...) but IIRC the "weird duration so it is a ritual" justification is in the Watching Ward itself. So "no weird duration = does not need to be a ritual" comes directly from that 
I think it is not stated as clearly in the watching ward, though, and that this is part of the spell breakdown by components in the base of the spell description.
This whole issue opens a lot of potential for Moon duration watching wards allowing for instant spells carried by grogs.... And for amazing botches when the grog carrying the "grenades" (to put up an example) casually throws the bag carrying the wooden balls enchanted with POF into a cart. FIREWORKS!!! 
That would make the fast items of the natural magicians worse than a magus with watching ward and a good spell repertoire. In half an hour of not very heavy work, a magus can have 50-ish spells ready for his grogs to go out with. Bring a disenchant item as well, though, or you can have quite a mess in one month's time 
Cheers,
Xavi
Yes and no. Firstly, you need to have spells of a sensible level to go into the container (and for a nonVim specialist, getting a high level container is nontrivial), and you also need to have relevent spells - anything with a range of voice or sight is pretty useless since the triggering conditions aren't that versatile. So yes, it does allow for some potent combat effects, but also requires that you spend a while inventing sensible spells to use them. A collection of marked rocks and a magus Wielding the Invisible Sling does work nicely though. Botching an attack roll with an arrow set to go off when striking a target is also a poor idea.
As for the disposal - in my last saga, there was a very deep pit behind the covenant. The grogs did not go near it.
The best use of such a thing, however, requires some other Vim spells to hide one's Sigil. Then simply place Moon duration containers on a large number of mice, place large fire spells into the containers and then release them in one's enemies covenant or grain stores. Kidnapped grogs hit with a PeMe spell also work nicely.