My 3yr olds been getting up a couple times aweek in the middle of the night and getting out toys, taking off diaper,etc....
I don't know of any monitors, sorry.
Good luck!
I asked this question among lots of message boards. The monitor would never pick up anything because he would shut the bedroom door behind him. Many suggested bells---like "cow bells" hanging on the door knob. Well---we didn't have door knobs on his door---he had french doors with handles.
He grew out of it before we actually figured anything out.
But what I know now is that what works the best is an alarm!! We had gotten alarms for windows/doors. When he got older--about 3-4y, he would get up during the early morning and leave the house. He could unbolt anything we had. I went to Walmart and bought a package of these alarms. They work great! They just stick to the door frames and when they aren't in contact with each other--the alarm sounds----and you WILL wake up!!!!
Walmart might not have it anymore--but it is made by First Alert.
http://www.ekitchengadgets.com/fialwidoalsy.html?CS_003=7404 77&CS_010=fialwidoalsy
They do have those at wal mart still. I bought some not to long ago for our doors and the fridge (chris likes to get up at 2-3 am to sneak food, etc.) They do work great.i put a lock on my fridge when my son was 2 and he still learned how to open it.