She's probably not going to manage to follow him closely enough to see what he does in there while staying in the realm of 'safe following distance'... But she can go over every inch of it later, and at least try to get a glimpse, either through the door if he leaves it open or through one of the high windows.

Unfortunately he closes the door behind him.

She scrambles for a window.

She'll see him open a stone trapdoor designed to look like a weathered old tile, descend a ladder, and let the trapdoor close behind him.

She makes a point of memorizing where the trapdoor was, then heads back to Giles.

"Any news?" he asks as the car door thunks shut behind her.

She relays what she saw.

"Hmm," he hmms.  "That could be Vanessa's hideout or Galbraith's - but in either case we can return here with more supplies and conduct a proper investigation."

"Definitely. And backup, like Gaara."

"Indeed."  He notes down the address and starts driving away.

And if he needs nothing else from her, she should go find Gaara.

Maybe after sleeping...

He does not. "I'll take you home and you can get some rest," he says affectionately.

Nod. "Thanks, G-man."

Supplies are gathered, plans are made, and the next day - while the sun's out, so Galbraith won't have anywhere to bolt to - it's time for a break-in.

She and Gaara are ready to go. Gaara's taking point on scouting, being sneakier than Nausicaa, while Nausicaa hangs back a bit.

Gaara will find the cemetery, and Galbraith's crypt in particular, deserted.

Then time to use her sand to investigate the trap door.

It blends in remarkably well with the stone floor, but if you know it's there, and roughly where it is, and happen to be made of sand, it's not hard to find it.  It latches from the inside, but it's not airtight: she can get through with time and patience.

She has those, fortunately, and it's presently the morning. She goes slowly enough to be quiet.

There's a ladder, leading down perhaps ten or fifteen feet to a small complex of secret chambers.  It's not pitch-black but it's dimly lit enough for further scouting to hinge on the quality of Gaara's night vision.

Gaara is a sand demon not a night demon.

She'll step to the side and let Nausicaa take point, though she keeps her sand spread out, finely suspended in the air, to substitute for vision. It risks people noticing there's a lot of grit in the air, but it'll give them warning of things even past their line of sight.

Nausicaa is probably not a night demon but has very good night vision nonetheless. 

There are a couple doors off of the little entrance hall.

Presumably her sand can slip through the cracks - her sensing isn't fine-tuned like this, but she'll get an idea if there's anything moving beyond them, and of vague room or corridor size out to about twenty feet.

Each door leads to another little room, not much bigger than the entrance hall.  A couple of them have closets, in turn.  It's a small place.  If Galbraith is here (and that secret door didn't look like it was latchable from the outside) he's not disturbing the air very much.