The thing is, Buffy isn't hiding from Spike. The fact that Spike knows perfectly well it's Buffy he's having fun with in his posh crypt doesn't concern Buffy at all.
She is hiding from the rest of the world.
Let's return to a scene at the time of another 'it's complicated' non-relationship of Buffy :
We're not friends. Never were.
I can fool Giles, I can fool my
friends, but I can't fool myself.
Or Spike, for some reason.
~Season Three - Lovers Walk
Season six Buffy is in deep depression. Her friends returned her from heaven, and she is not taking it well:
BUFFY ... I think I was in heaven.
Spike stares, takes it in. She looks at him.
BUFFY And now I'm not. I was torn out of there...by my friends.
Everything here is bright and hard and violent... Everything I feel, everything I touch... this is Hell. Just
getting through the next moment, and the one after that... knowing
what I've lost...
She's said too much. He's speechless as she rises,
starts out of the alley. She pauses, not even looking back.
BUFFY They can never know. Never.~Season Six - After Life
That's essentially it. Season six Buffy is miserable, and for the first time, she is truly isolated from the Scoobies. Her friends dragged her out of heaven and she is suffering, but she doesn't want them to know how deeply miserable she is. Spike - well, in Buffy's opinion, Spike can take it. And anyway, Spike sees through her (forgive the pun), she never could lie to him. He can also engage in slayer-style recreational activities, Buffy doesn't need to hold back or pretend. And at least she can feel something. As she sings in their duet:
"This isn't real / But I just want to feel"~ Season Six - Once More With Feeling
What they have is an unhealthy affair Buffy is ashamed of, because she is the Slayer and he is just a soulless demon with a government chip in his cranium:
from here - spoilers beyond the episode the question refers to
Buffy: You don't... have a soul! There is nothing good or clean in
you. You are dead inside! You can't feel anything real!
~Season Six-Dead Things
and she is afraid that her friends, Giles and her little sister will find out what she does at nights when she is supposed to patrol the graveyards to protect the citizens of Sunnydale.
So while Buffy is struggling with her depression, Willow's addiction, social workers, a resentful Dawn, unemployment and bills beside her Slayer duties and trying to hold it together and not think about her guilty affair with a persistent vampire - he drops in. In bright daylight, when these things are supposed to be hidden. Not the best timing, and he unsurprisingly gets thrown out. At this point, it's less about regret and more about keeping the affair secret.
In season six there is a conflict between Spike, who wants to tell the Scoobies about their relationship, and Buffy, who denies that there is a relationship in the first place:
SPIKE: What is this to you? This thing we have. BUFFY We don't have a
thing. We just have... this. That's all. SPIKE Do you even like me? ~Season Six - Dead Things
So when she gets hit with the Trio's invisibility ray, Buffy finds the situation hilarious. No one can see her, so she can do whatever she likes. She doesn't have to pretend to be the perfect friend, sister, and guardian. She can play with Spike with Xander(!) in the room. There will be no consequences. She feels free:
INVISIBLE BUFFY: No! Maybe because for the first time since ... I'm
She tosses the sheet aside. Spike looks around, trying to figure out
where she's going.
INVISIBLE BUFFY: Free of rules and reports ... free of this life.~Season Six - Gone