I use the breeder alert (the box that hangs under the halter and sets off a pager when on its side), along with cameras. You can get false alarms, obvously if your mare lays out flat to nap, which doesn't usually happen too often, but some other ways are if your stall walls are short enough for them to rest their head on. My foaling stalls are tall enough they horses can't rest thei heads on and feel like they have privacy, but short enough they can peek over to see the other horse. Now and then they can also go off if you have wall mounted feeders and the mare catches it on the feeder or water bucket. But that isn't often and usually only lasts a minute or less. I also have one specific mare that will lay down, sitting up and rests her head on her front legs in a way that turns the box sideways and sets off the beeper, but she is the only one.