Bloodseeker has the Attach ability.
Attach: When a bloodseeker hits a target larger than itself, its barbed legs attach it to that creature. This is similar to grabbing the creature, but the bloodseeker moves with that creature rather than holding it in place. The bloodseeker is flat-footed while attached. If the bloodseeker is killed or pushed away while attached to a creature it has drained blood from, that creature takes 1 persistent bleed damage. Escaping the attach or removing the bloodseeker in other ways doesn’t cause bleed damage.
Grab causes the grabbed creature to be flat-footed, How about if a creatures is Attached?
I'd say yes, the target is flat-footed. The intent seems to be that the target gets a variant of the Grabbed condition except not being immobilized (and having the grabber attached).
I think calling the having-a-creature-attached condition only "similar" to Grabbed and not Grabbed with extra rules is to have it not work with other special attacks that require the target specifically Grabbed.
I don't think making the target flat-footed was intentional, yet that's how it reads.
As Masda pointed out, Attach does not count as Grab for abilities that require a Grab, but otherwise works like a grab w/ only the one exception that the target can move. If that's the only exception, then the target is also flat-footed as usual.
Paizo may have referenced Grab for the ease of understanding Escape, and forgotten about the other aspect. I'm still considering whether having a meaty bat stuck on me would distract me enough to make me flat-footed to other threats...