Intel Core i5 (four cores) running OS x 10.10.3 and Boinc 7.4.42. Activity monitor shows the bonic process, running under boinc_master, using a LOT of cpu time. When no boinc tasks are running, it still uses 100% CPU. When four boinc tasks are running, each of the five processes are using 79%.

This happens on both of the iMacs. The Windows 8.1 machines does not show such activity.

What could be going on when no boinc tasks are running? A new feature? A bug? Or have I been bitcoined?

As I was pressing the post button, I said "reset". Better yet, reinstall. I emptied the work queue and reinstalled just now. Now it is all ok.

Mostly OK. My passwords are now invalid. I was able to reset yours while writing this question, but the others were not so lucky. So it's off to another machine to try again. Could it be that you invalidate passwords if not used in X months?

I just noticed that boinc is 100% on mac osx 10.9.5. I even suspended it when I went on battery.

Boinc process still 100%, it's constantly doing this:
madvise(0x6C7A000, 0x80000, 0x9) = 0 0
mprotect(0x6C7A000, 0x80000, 0x3) = 0 0
sigreturn(0xBFFFF458, 0x1E, 0x3) = 0 Err#-2
madvise(0x6C7A000, 0x80000, 0x9) = 0 0
mprotect(0x6C7A000, 0x80000, 0x3) = 0 0

As I was pressing the post button, I said "reset". Better yet, reinstall. I emptied the work queue and reinstalled just now. Now it is all ok.

Mostly OK. My passwords are now invalid. I was able to reset yours while writing this question, but the others were not so lucky. So it's off to another machine to try again. Could it be that you invalidate passwords if not used in X months?

Looks like you could have set a value to 100, check out:
http://setiathome.berkeley.edu/forum_thread.php?id=77784