The same thing happened to me last night!
I was getting ready for bed and my wife went to check and lay down with our 12 y/o son, when I got a text message at 1204 am say "OK sounds good' from my wife's cell phone, so I text-ed back 'HUH'. I told her to come to the bedroom and I asked her what did she mean by the text? she said I never sent you that text, so we compared her phone and my phone messages and sure enough, that message was inserted into our previous conversion somehow, but it was absent on her phone and only present on my phone? that is so wierd, its like someone was listening to talk, because I told her verablly in person moments before heading to the bedroom that I was going to bed and she said she was going to lay down with our son for a few minutes.......thats it, then 1or 2 minutes later I get that text saying "OK, sounds good"? that really creep-ed me out.