I can see how, if pre-planned, that our own club field could be vulnerable to thefts though this is more likely to be during the night as a break-in of our club portakabin, where the beginners equipment is stored along with bosses, stands and faces etc...... However, We are fortunate enough to be situated on an old cricket field which is within a village and is gated and overlooked by nearby houses.
We minimise the risk during the winter season by moving the beginners equipment up to our indoor range which we are also very fortunate to have located on a disused air base which is owned by a local development company. It still retains some operational importance and is still an MOD concern with the security still in place (civilian not military) requiring the production of a pass to gain access to the site.
Having a remote club field is always going to be a headache for security but there are things that can be done to minimise the risk. Using strong locks and shackles on containers, huts, buildings etc. removal of valuable equipment to a safer location during the winter months (not always practical if alternative storage cannot be found, I know!)
Regular patrols by members that live nearby if practical.
Most importantly, let the local police know that you have archery equipment secured at the site. I'm pretty sure that there will be more patrols if they knew that there was something worth protecting as I'm sure that they would not want such things falling into the hands of the local criminal fraternity!
If, Even after all this, the unthinkable should still occur, then measures like the use of U.V. marker pens and smart water will go a long way to help aid recovery.