What is the DC to notice the portal for a mirror of mental prowess?

[seebs]

The rules say that the DC to notice a scrying sensor is "DC 20 + the level of the spell".

The mirror of mental prowess says that creatures with an intelligence of 12 or higher can notice the invisible portal, the way they could notice a scrying sensor, but doesn't give a DC. They might be implying DC 21, but that's ridiculously low, since it's obviously not a first level spell. The actual spell could be either of clairvoyance/clairaudience, or gate; I'd be inclined to say that the portal counts more as gate than as clairvoyance, maybe. I'm pretty much ruling out detect thoughts and legend lore as irrelevant, but that might get you DC 23 or 29.

But I don't see why the "intelligence 12" is mentioned, since perception checks aren't intelligence-based, and I don't see a statement of DC there.

EDIT: The int 12 thing is an editing error. 3.5E D&D's rule was that creatures with an int of 12 or more could spot a scrying sensor with a DC20 int check. The rules for noticing scrying sensors were changed, the mirror wasn't updated. Pretty sure the "intelligence of 12" should just be ignored.

[Lincoln Hills]

Yeah, that's fossil-text. What I'd probably do is make it a Perception check to notice it (exactly as if it were a scrying sensor) - and then require a Spellcraft DC 29 check (to 'recognize the effects' of a 9th-level spell equivalent) to realize it's not just somebody scrying. That is, assuming the party barbarian doesn't try to put his tongue on it or something: that would obviate any need for a check.

[seebs]

One of the things I want to do with it: Place the sensor in front of someone who's running. Open the gate right as they are approaching. I figure if they run through the gate without being aware that it's there, that's a good way to lift someone out.

(There's a long story here involving plans for an intelligent item mirror of mental prowess, which could in theory open and close the gate for us on command, but if you're habitually mind-blanked the scrying device can't see you to determine that you want something.)