Mrs. F isn't at all that tech savvy. I bought her a phone but she never uses it. I have old phones and sims all over the place at home so finding my always wiped punting phone in my car even is she did would not be a problem. Even if she did find out it had a working sim it's just my backup, communication is crucial in my job.
She's no fool though, so if I caused her to become suspicious it might call for a change, it's just not possible to be sure I won't make a mistake. Bottom line is if you can't live with the risk, don't do it.