Pequeno Burraidh
Beag Combatiente
Uh-huh. Sure it is.
Certain characters, I imagine, would know right away if they've been marked with chakra. Baihua for one, since she has the byakugan. Sharingan users will probably be able to tell. Chiaki can smell chakra as a byproduct of being a Wolf Sage. Skilled sensory ninjas like Jun would be able to tell. Both Shinsei and Raina carry chakra/spirit detectors.
Nao visits when he can, because of the abundance of sharks, but he probably spends more time in Kiri's beach than the bay of another nation. I'm glad Rei marked Nao then. Though, it's not like Nao would lose to shark. He's a ninja that can turn his body to water =P
Certain characters will know right away that they've been marked.
Ah, but she didn't mark the leaders with chakra, she just moved sand that is infused with chakra onto them. I probably should have been more specific on that. Actually marking someone is where she puts her chakra either on you or in you, basically infusion. Her whole sand thing? Well that's just so she can find herself to the location of anyone she's bugged, so the sand is infused, but the person the sand is on isn't. That said Rei can still summon herself to them, though if Tainan were to have, say, thrown his sash into the dirty laundry before she tried to summon herself to him, she would have come out of the deal with a pair of knickers on her noggin.
Up until it dried, the water on Kauri also had him bugged in it's own sense.
As for characters that would know they've been bugged with sand, well that's up to their writers onhow they handle that. I'd imagine Baihua would find the action interesting given Rei's non-violent and peaceful tendencies.
Sides, do you really think Rei bugged people for bad reasons? Pfft, please, outside of a well deserved kick she didn't even hurt Masima and he tried to kill Rokoriko.