Building on that, some battery powered GPS units these days are getting relatively cheap and can be used to stream location data. My guess would that in the event of a disturbance, the horses would be much more active (moving around) more than one might expect? Integrated with the above; it strikes me that you need some 'disturbance-phenotype' to detect for. IE, you can get the data-loggers and instrumentation onto the beast, but then you need a way of defining 'disturbance'.

Maybe once you've got that definition you can set up some spotlights to come on automatically once a certain level of 'disturbance' is met?