Noticing the sensor of a scrying spell

[Whimsical]

The "based on INT" is a carryover from 2nd ed, where creatures with high HD and above average INT could see scry sensors. Which is why the DM's BBEG always knew that you were scrying him, and therefore nerfed that spell to stop you from "cheating."

Personally, I think that Spot would be the mechanic to use, since it can already notice invisible things.

 

[Christian]

The "based on INT" is a carryover from 2nd ed ...

And then a further carryover from 3.0, in which a Scry skill check was used for this. For unskilled characters (i.e. most characters), this would be a straight Int check.