You don't need to overcomplicate that one. You are literally invisible. After finding a nice lamp post to hop the fence from, you land in the barren, depressing-looking dirt field around the building and get to crouch-walking. The only interesting thing you got out of that experience was finding some sort of trapdoor or a hatch seam in the ground. You didn't try prying it over, who knows, maybe it's a turret or something.

With rain masking every sound you make, getting to the concrete slab and climbing it wasn't a problem at all. There's no roof access at all, and you only found one window, which has a pretty dingy-looking office behind it. Did nerd just lie to you about the location? You work some thin mycelium strands into the window crack and slowly, painstakingly get it open.

After looking through some empty, dusty corridors, you find a bunch of heavy-duty doors leading deeper into the building. Breaking those open would probably be way too noisy, plus you found a better idea. An all-time classic.

A human would probably have to break a lot of bones to fit into the ventilation duct you're crawling through, but with your formless body, and with only hard part of your shell being the helmet, you squeeze in nicely. After mapping the ducts with mycelium, you find a way deeper and use all your stealth game knowledge to proceed. You avoid a bunch of sensor-looking things by exiting the vents and re-entering at a different point. You avoid cameras by stuffing yourself into the vents. You avoid one (heavily armed and bored to death) guard by just not going in his direction. It's a pretty uneventful and slow infiltration, but you don't mind.