And the corp wage mage assenses the watcher, and sics a bunch of spirits to track down whomever created said watcher (or the resident spirits that are patrolling go ahead and blast the watcher before it becomes a nuisance at all). I think using a watcher like that would be tantamount to throwing eggs at the corp's HQ, with a lot less likelihood of getting away.

Any competent security manager would be ready for such a tactic; if someone kept pinging a security alert, they'd be determined to find out why their alert was constantly going off, and fix it so it stopped (by thwarting a DDoS, tracking down the offenders, or setting a trap for whatever vandals there are), or adjusting their security parameters to account for whatever's causing the problem.

The squirrel setting off the motion sensors is a good one. It's only slightly suspicious (where the frag are all these squirrels coming from?), and could cause them to either 1) adjust their sensors to ignore squirrel sized objects (and thus allow sizeable drones to infiltrate), or create complacency over the nightly triggers. Or maybe they make a game of it to bag the squirrel; i.e. it backfires, and security is watching more closely at about the same time, night after night.