"Motion Sense" needs more clarification

[hyphz]

Several monsters are listed as having "Motion sense", including an extremely dangerous encounter on the Age of Ashes Adventure Path.

But the only description of motion sense is a single line on page 254 of the Bestiary, under the "Sewer Ooze", which is the first creature with this ability.

It does not even mention whether or not Motion Sense is an imprecise, precise or vague sense. There is no default given in the main rulebook and the sense doesn't have (imprecise) after it. For monsters with no vision this is potentially a big deal - if they have only imprecise senses then everything they do has a 50% miss chance due to Hidden status, which dramatically changes their balance.

Plus, we don't know what motions it can sense. Is it just Move actions, or can it sense lips moving to speak, chests moving to breathe, heartbeats? All make a massive difference to strategy.

[thenobledrake]

it's not marked as imprecise, so it isn't.

This sense appears to be a means of seeing without seeing, but also without including various other bits of baggage that terms like "blind sight" might carry.

It can sense the motions mentioned: things that cause vibration and/or air movement.

So hold your breath and stand still and it can't "see" you.

[Claxon]

Yeah, I take it as a precise sense that can detect virtually any movement. Though it does bring up the question whether or not the motion of your heart pumping blood qualifies?

For it to be fair, I would say anything that involves any body movement and requires an action is able to be perceived. So breathing and heart pumping wouldn't be detected.

[SuperBidi]

I would allow a player to Hide against motion sense by holding its breath and stop moving. One action and a Stealth check. Considering the intelligence of oozes, it's equivalent to being completely undetected, so I think it's a fair ruling.