I haven’t figured out the exact issue yet, but ever since I (a) switched to Mac OS X 10.9 and (b) added a second monitor to my old iMac, the mouse pointer/cursor seems to disappear from time to time. Actually, I can’t tell what it’s doing, so I move the mouse around frantically until it reappears.
To help resolve this problem, I just made my mouse cursor larger. You do this on your Mac by going to System Preferences, then Accessibility, then Display, and then adjust the “Cursor Size” slider until you get the mouse pointer size that you want. You can see in the image shown here that I’ve made mine a little larger than normal. (I was surprised that this is under “Display” and not under “Mouse”, but there it is.)