How can I find out when a USB device was disconnected without ejecting? I had a USB flash drive plugged into my MacBook (running Yosemite), but that drive is now missing. Somebody must've taken it (unplugged it) from my computer while I was at work or while carrying it in my backpack.

When I now opened my MacBook, it showed the message that the disk wasn't ejected properly.

Can I find out when it was unplugged? What sort of message am I looking for in the event log?
Try the Console log at var/log/asl/2015.04.08.U0.80.asl or similar
(there will also be a G80 &amp; a U501 with similar dates but it's the U0.80.asl you want)

Apr  8 17:38:35 macname kernel[0] &lt;Notice&gt;: disk6s1: media is not present.  
Apr  8 17:38:35 macname kernel[0] &lt;Notice&gt;: msdosfs_fat_uninit_vol: error 6 from msdosfs_fat_cache_flush


Tested empirically on a disposable USB key ;-)