Scrying In Pathfinder

[Chambers]

The Perception DC to notice the invisible sensor created by a scrying spell is 20 + the spell level. I am running a Pathfinder game and have found this DC to be laughably low (the party is 11th level). Its trivially easy for these characters to notice they are being scryed upon as soon as it happens and though this also applies to the enemies that the characters are scrying on it seems a weird spot in the rules.

Are there any RAW ways to increase the Perception DC to make to harder to spot or should I make a table rule that increases it across the board (like adding the spellcasters caster level to the Perception DC)?

[dude123nice]

This seems like a fairly decent inherent drawback to a pretty overpowered tactic.

[Psyren]

The scrying sensor is invisible, and nowhere in the scrying subschool does it say this condition is already factored into the DC. So RAW, you should really be adding another +20 to the Perception DC per the Perception skill rules (to make the final check a minimum of 40+spell level) unless the PCs have an active method of seeing invisible things. And if the PCs are in combat or sleeping, the DCs are raised to nearly impossible levels.

This doesn't apply to the 3.5 version since there, you weren't using Spot to try and notice the sensor, you were making an Intelligence check, which is not modified by visibility.