A Flash Washer is someone in your office that briefly runs their hands (sometimes just fingers) under the water after using the restroom. Basically stating that their restroom activity did not warrant a soap and warm water hand washing.
I'm not eating anything that Donna brings to the pot luck on Thursday, she's a Flash Washer.
A person whom only washes their hands after using the restroom in the visible presence of others. If it "appears" that nobody is present (i.e. somebody is in the stall, or they didn't notice someone was in the room with them), or nobody is actually in the restroom with them, they will leave after using the restroom and not wash their hands.