Since there hasn't been much SHIELD discussion about their observations on the monolith it is really hard to say. There are plenty of options. It could possible...
a) Sploosh in front of anyone, everyone, or nobody, which may or may not be part of a pattern, apparent or otherwise. (in which case it is hard to imagine SHIELD knows much about it).
b) Sploosh in front of transformed Inhumans only (which would seem to require there to be an anonymous Inhuman among the SHIELD agents in the scene it splooshed when only SHIELD Agents were in the room).
c) Sploosh in front of transformed and/or non-transformed Inhumans only, unless it touches a human (in which case their could be Inhumans among SHIELD that don't even know they are Inhumans).
d) Any of the above, but with the added clause that only Inhumans actually perceive the change, whereas regular ol' Humans do not perceive any change. (could negate the need for an anonymous Inhuman SHIELD agent in the guarding scene if the Sploosh was for our benefit...)
Any of the above would certainly peek SHIELD's/HYDRA's interest. D might have been hard to SHIELD/HYDRA to even know about, depending on their make-up of agents. Even if they didn't have any (known or unknown) Inhuman agents we do know SHIELD (or HYDRA within SHIELD) had Inhuman prisoners they did experiments on.