So I’m trying to pump myself up for sneaking out of the building without any coworkers seeing. I’m going to go over to another building to do some important things that are not work-related! I’m scared! But it’s exciting! I like living on the edge, man. 
Remember, you have to dress like a cat burgular and walk on your tippy-tip toes and make “sneaky” noises (kinda like a xylaphone).
Good Luck, Intrepid Quazz!
Just look kinda worried as you walk out. It helps if you’re holding a clipboard and the exit is near a department that you normally frequent.
Of course, if that’s not adventurous enough you could always braid a rope out of rubber bands and bungie jump out your window. 
Don’t forget to keep a cyanide pill handy. Just in case you get caught.
::Cue Mission Impossible theme music::
when I was a kid I used to get into my school to cause mischief when I wasn’t meant to be there (like at weekends) using this method: walk with a purpose, eyes fixed ahead, hopefully carrying some sort of object (clipboard, as suggested, is good). If anyone asks you, you tell them that you’ve got a “special project” meeting. Bada bing.
He hasn’t posted in two hours.
Either he made it to the other building, or has been captured by strange men in black suits and sunglasses.
Just leave your desk messy and it will look like you are busy.
Everytime my boss is out of town I leave a half hour early & pray that no one notices. But since I am the only female in the building its kinda hard not to be noticed gone(but I do it anyway)
Do some James Bond stuff, like jump through a window at the top of your building, then pop a chute with a big ol’ flag on it and just gliiiiiiiide to the other building.
And don’t ask the secretary to hold your calls; she’s disavowing all knowledge of your actions…
Well Qazzz did you sneak out? How did it go?
Yes, how’d it go? Inquiring minds want to know!