Warnings: Spoilers for The Great Game, some sexual references and one use of bad language right at the end. That is all ;)
All this came from Sherlock's deductions in 'A Study in Pink', particularly the line: "…and then she scrubbed your floors too, judging by the state of her knees" which never fails to make me laugh.
I do not own, as I am neither Stephen Moffat, Mark Gatiss, or the BBC.
Sally peered over the top of her computer, her eyes fixing on the dark hair a few desks away. Not for the first time, she wondered why she did it. His wife was to be home for a good few months now, and she knew the stabbing disappointment well: every time, every damn time, she hoped that maybe she wouldn't come back. Not in a horrible way: she didn't want her dead or anything, she just hoped that one day she'd find out, realise that she could do so much better. Really, she could. Sally had met her once: a small blonde woman with a cheerful smile and small crinkly eyes, a few years Anderson's junior. She didn't deserve to be treated how she was, and every time Sally thought of her she felt a stab of guilt mixed with that disappointment.
Realistically, it was her that should be looking to do better. Anderson was married. She was not unattractive, she knew, she was still capable of getting attention if she wanted it, but for far too long, the only attention she had wanted was his. Every time he came calling, she would follow blindly. She didn't know why. He wasn't the most attractive, he wasn't rich, he wasn't faithful, and they didn't have a future. Yet he held a strange kind of allure that she was utterly powerless too. It wasn't love, she didn't love him, but equally, she couldn't stay away. That made it worse. Love would have been an excuse.
As it was, he had complete control over her. She sighed. There was only one person she'd ever met who behaved quite as stupidly as she did, and she'd only known him for a few months. She leaned back, moving her eyes from the back of Anderson's head, and thought briefly of the mild, light haired doctor, who'd stumbled onto that crime scene that fateful evening, and had never really left. Tapping a pen impatiently on the desk, Sally realised he hadn't really understood her warnings: 'Stay away from Sherlock Holmes' and 'He's not your friend'. She hadn't done it out of spite, she'd seen herself in him. She knew, from experience, the danger of responding to one man's beck and call, following him wherever he asked, not really sure what you meant to him. That was her and Anderson. It hurt. She'd wanted to prevent John from falling into the same trap, but she knew, really, that she'd lost as soon as she saw the hurt in his eyes, because Sherlock had left without him. He'd fallen already. Just like her.
In many ways, Sally was insanely jealous of what that pair of idiots had. Well, she couldn't quite put her finger on what it was, exactly, but she wanted it. It hadn't helped her like Sherlock anymore. She didn't actually believe they were a couple, whatever she teased them about – she knew what couples looked like, and they never included Sherlock Holmes – but they had an understanding, a sort of closeness, even as Sherlock insulted everyone in the room, and John dug his hands deeper into his pockets and cleared his throat. There might have even been love in that relationship. The look on Sherlock's face as they'd dragged him and John out of the wreckage of that pool was not one she'd thought him capable of: his eyes never leaving his flatmate. He was shaking. John, on the other hand, had stubbornly refused all treatment until Sherlock was seen to.
She and Anderson were not like that. She and Anderson were complete strangers for months on end, then as soon as his wife left on business he'd take her home. She was the dirty little secret, the quickie in the back of the police car, the wandering hands under the desk. It wasn't fair. No wonder she could be so bitter. Annoyed at her weak will, Sally closed the window on the computer rather more viciously than necessary, packed up her things, and left. Well, not without one quick glance at him through the glass. Stupid bastard.