Using the ObjectSearch I usually presss F4 to describe the object found...
Why is the F4 not working...? 
Then I noticed that the button for "describe" is shadowed:
Why can that happen? Is there an option I haven't found yet?
Thank you for reporting that. It's a bug. I can reproduce that and will fix it. Looks like it only happens for package bodies.
You probably know this, but as a workaround, now that you know which package you are looking for, just do a CTRL+D, then type in UTILITY and hit ENTER