If someone with an FRS radio was close enough to hear them then they could have just picked them up. One of my counsins bough a pack of them one year for deer season and you could have written you message on the radio and tossed it at the other person sometimes when they wouldn't pick up each others signals.
GMRS is a little too application specific so people are not just driving down the road listening for them.
Your much better off broadcasting a signal so HAM or even CB since there will be more chance of someone hearing it