Kay calls her when she's eating the pizza in the car.
"Been better"
"Everything seems to be going great with the library, is everything cool?"
"Not really, that's why I'm calling...
Things have been really shitty round here.... I finally know what's been going on"
She phones her when the access code doesn't work, Kay says hey, hear you go. she's still connected with her and the access code comes through on text. She goes inside, shouts Kay twice, then Kay comes through the Skylight, screaming. There's a clang like a scaffolding pole beforehand.
Interestingly, there is a scene just before this in the office, where Kay is looking at Paula suspiciously, from across the office, she's oblivious, but she then looks at husband, who is oblivious. So set to look like Kay caught her looking at her husband maybe?