If you are just trying to find Talia Brooks I suggest you go to the security station and enable tracking for her - if however this does not work she may have despawned. The body of Talia Brooks can despawn from the coolant room taking her story critical keycard with her. If this occurs I recommend the following steps:

1. Save your game outside of the power generator map, life support is fine. Quit the game.

2. From the steam game library right click on Prey and go to properties. Click the update tab and at the bottom of the menu uncheck “Enable Steam Cloud synchronization for Prey”. This means the saves will be stored on your computer - if you tick this box from now on, steam will overwrite your current saves with the outdated ones without prompt!

3. Go to C:/Users/(your name)/Saved Games/Arkane Studios/Prey/Save Games/Campaign (whatever # it is)/ and delete the "powersource" level file from both your save and the temp folder.

4. Reload your save and enter the power reactor map which should have reset with all it’s items, enemies and most importantly Talia Brooks back at their starting positions.

This fix is not officially endorsed in any way. It worked for me, who knows what it might do though?

Thanks to antonius66 for the fix. Original thread here: https://bethesda.net/community/topic/53586/talia-brooks-key-body-vanished-in-reactor-room-i-fixed-it-myself-see-within-if-you-re-having-the-same-issue/18