None of my coins overseas are missing, but I have many MIA here in the States. Two of them didn't make it past their first or second cache. If I ever find anyone stealing travelers, you can bet that I will take action to resolve that situation.
Don't let it keep you from sending some out though, b/c we cachers really do enjoy moving them. You just have to set it free with the mindset that it's probably going to get stolen or lost along the way. At which point, you can make a custom TB and reuse the tracking number. Just be sure to rename the "coin" accordingly, so folks don't get aggravated.