The First Clue
Lena's had a suspicion that her soulmate compass is defective for a long time. The first clue was that it didn't appear until she was nineteen years old. Since no one gets their timer until their soulmate appears, lots of people don't get one at birth but when they are an infant or toddler or even in elementary school. She supposes that half of everyone has one at birth and half get them afterwards. But nineteen? She's never seen herself as a cradle robber. Lex thinks it's the funniest joke he's ever heard. He got his at birth and of course he couldn't care less to find his soulmate anyway. But nineteen? Like when she's 37 she'll meet some 18 year old girl and want to hook up? That sounds unlikely even for Lena. Lex says maybe when she's 34 she'll meet a really hot 15 year old and she slaps him for that one. He keeps laughing.
The Second Clue
Lena's second clue that the stupid compass is defective comes shortly after she moves to National City. For years the compass hasn't moved which isn't unusual. For most people the compass won't begin to move until they are close to their soulmate since the direction of 4th street vs 11th street in a city 3000 miles away is negligible. For the first few months that she's in National City the compass moves very little. She suspects that perhaps her soulmate lives in National City, probably attends elementary school in the suburbs or visits the library with her parents. Only one day the compass starts moving rapidly. Very rapidly. It acts as if her soulmate is riding on a motorcycle. Or a race car. Or a jet engine. The needle swings from one direction to another in seconds. Even if her soulmate were in the room with her, she doubts she could run a circle around Lena's arm that quickly.
The Third Clue
The third clue that the compass is defective comes when two reporters come to visit Lena. Corn-fed Clarke Kent certainly isn't Lena's soulmate but Kara Danvers is quite intriguing. She's wholesome and pure and a little shy and has bright blue eyes that can't be dimmed by her glasses even if she seems to be hiding. And the compass on Lena's arm points directly at Kara even as Lena hangs up her coat and purse, pours a glass of water, and retrieves the data the pair came for. But Kara Danvers has no compass on her arm and she's not acting like it would be pointing at Lena if she did. And Kara is certainly more than nine years old.
The Fourth Clue
The fourth and possibly strongest clue that the compass is defective comes when Lena is almost killed as her helicopter takes off from the roof. As she watches Supergirl in horror and fascination she can't deny that now her defective compass is pointing directly at the superhero. Lena knows plenty of polyamorous people but as far as she knows no one actually has two soulmates.
And anyway, Lena Luthor is fond of showing the universe that it's not the boss of her in any way she can.