We haven't had any false alarms with ours and have been using it since DD was 10 weeks old. We have the model with the two sensor pads. The only times it's gone off is when we've taken her out of the cot and forgot to turn it off

They are extremely sensitive though so like the PP said, test it. A fan works well, we found that out the hard way - our fan was blowing on DDs valance so the monitor detected movement Yes, they are very sensitive. We took off her valance and stopped using a fan (or pointed it away from the cot) for this reason.

There is a possibility the monitor did it's job for you, as scary as that thought is. Give it a good test run using a fan and adjust the sensitivity and hopefully you'll have a better night tonight.

Good luck.