You’re actually asking two questions here:
- How to reliably detect a prowler in close proximity to your camp,
- How to alert yourself to that fact promptly but without the prowler knowing that you’re onto him.
For the first issue you want a detection system that’s quick and easy to set up, and take down, while at the same time having a high rate of successful intruder detection and a low rate of false alarms.
Here I would suggest a PIR that clamps onto a 1 inch tent pole and looks downward onto the target area. This unit can quickly clip onto an upper and outer tent pole while its trailing cable plugs into a receptacle on the outer wall of the caravan. The whole setting up takes just a few seconds and its just as quick to break down. Any device that requires complex or time-consuming setting up will fall into disuse sooner or later.
A downward looking PIR, or one that looks downward at an oblique angle is very effective at protecting an area that is smallish and that you want to keep confined. Mounted to look horizontally the range can vary but will likely be bigger than you’d want, given that there are neighbouring campsites and the access road nearby, but when you face the PIR downward, or near downward, the edge of the observed area is quite easy to determine and also easy to limit by using strips of shiny tape at the edges of the lens.
Also the ground that the PIR is looking at is incapable of generating false alarms since no heat source can spontaneously arise there, but small animals will be detected, which may or may not be desirable. You may for instance want to be alerted to the presence of a mongoose or a monkey, but not the movement of your little dog.
The second part of the question is how to be alerted to the detected intruder? Since I know of no competing product I’ll use our own as a generic solution. We have 3 ways of alarm annunciation – a built-in non-adjustable piezo buzzer that has a muting switch, a mobile phone screen display with accompanying sounder which can be adjusted or silenced, an SMS sent to any mobile phone that displays relevant data and produces whatever audible and vibrator output the phone has been programmed for by the user.
It seems to me that in your case you’d want to mute the local buzzer since that will most likely be audible outside the tent in the quiet of the night,
Using the screen annunciation would work well if you expected an imminent intrusion, but would require the app to be running in the foreground which would increase power consumption and shorten battery life considerably.
For non-imminent threats i.e. those that have a moderate to low probability of occurrence the SMS option would be best. It will take just a few seconds for the SMS to travel through the network to your phone, and you can set your phone up with the combination of audio and vibrator response that suits you best, without conveying anything to anyone outside the caravan or side tent.