Yes, I do think its possible, and this is a huge fear that I have.

When my Mom-in-law was very sick, they set up an intercom system (very similar to a baby monitor) so that she could talk to my dad-in-law when he was working in the basement (he's a tailor). They would often get interference with the Dr's office next door, be able to hear the nurses talking etc etc. Also the dr. told them that he could hear their conversations and thought it was cute that they said "I love you" so much over the monitor.

I am very careful with our baby monitor. I "tested" it out by walking down the road and I got very clear reception all the way over at the neighbor's house (nearly an acre away). So I only use it when Lydia is asleep, am very careful what I say when it is on, and turn it off as soon as she wakes up or I don't need it anymore.