She still can hear and/or smell you (as darkness predator yokai).
In PvP shooters (where possible) I used as quirk shooting through "soft" walls using sound and prediction where target could be go. Chances increased if game have damage feedback. People even after months tend to yell "haaax!!". Here Sakuya surprised while barking aloud, running around in hard shoes and, more importantly, having smell of blood ...
...or this Rumia can see through her own darkness.
Well, she is canonically able to make it just bright enough for her to see in the darkness, while still blinding people not used to it, meaning just about anyone else (BAiJR). Even in EoSD, she was able to tell Reimu was the one she fought in the middle stage, while Reimu said she couldn't recognize Rumia because it was too dark.
Even putting canon aside, the artist is making Rumia pretty awesome in this series, so anything goes i guess.
Seems like an easier explanation is possible. Since we know the 3 faries are there, they could be guiding Rumia while hiding their presence.
Whoa!Wha'?Don't tell me......she can see me?HmphEh?She's able to follow my movements.If you're just going to shoot at random......then get on out here.