This happens to one of my sims about once a week. I just click somewhere in my house or yard and they appear there. Sometimes it'll happen when they're downtown or at the waterfront, the sim in question will be up on a bridge or something, out of sight. It's a weird but pretty harmless glitch as far as I can tell.
I see it more often too, don't think I've ever had a report for it on the bug forum. So if you want to, feel free to report it:
I do not work for EA