This might be a stupid question, but...
We have one of those AngelCare baby monitors -audio as well as the pad that senses movement. It seems to have worked ok -we agreed to stop using it if we got lots of anxiety creating false positives, but haven't had any. It only goes off when we forget to turn it off and pick him up.
Last night, DS (14 weeks) was sleeping in his bednest next to me when the movement alarm went off. I woke up, just in time to hear him suddenly take a big gasp (like when you've been holding your breath) and the alarm to stop sounding.
Now, is this just one of those things, or is it something to react to..be concerned about?