Pretty grim situation, but interesting. It depends, the last desk job I had I was kinda the odd man out (unusual for me) so if I didn't have a buddy to spend the last few minutes in reflection with, I'd probably just try to find some place to be alone with my thoughts and away from whatever other calamity would be going down (which would no doubt be the case). I am not sure how much "trapped at the office" means, but if I could get outside, I would - preferably to see some actual nature, the sky as the last things registering optically for me, rather than staring at carpeting, ventilation piping, fluorescent lighting and office equipment. And then I suppose that would be it, just thinking about everything that had led me up to this moment, if I had led my life satisfactorily, or where there a bunch of regrets and things I'd wanted to get to/missed or blown opportunities. I guess my mood would be colored by that, mainly. It's really hard to imagine, because if you are legit faced with your own imminent demise, I am sure there's a giant rush of adrenaline pumping through your entire system as your fight or flight is maxing out. Also it is said that when an organism knows that death is imminent, it will do a hail mary of trying to reproduce with whatever energy still remains. Being self-aware of all of that I am sure it would affect my mood and actions as well. Maybe I'd go ape shit and just start screaming and breaking everything in sight. Who knows.