Please forgive the geographical bias in this, and therefore the cost element, but in most of the places that we fly in Asia as private pilots, if we are going to leave the aircraft there for a lunch-ish period of time we might, rarely, consider arranging a security guard to watch over it. In this part of the world it only costs a small proportion of the fuel bill, but your costs may vary...
It is different from leaving your car in the restaurant car park. If some mongrel lets your tires down and you don't notice, then your stop and curse and sort it ten metres later. If same mongrel pulls a bit off the tail rotor drive and you don't notice, your are not going to pull into a garage after airborne to get it fixed...
Depends on the country and the area. In Hong Kong the social conscience of the average at risk teenager is delightfully strong, and vandalism is very rare.
So I guess look at your area and make your risk assessment. Pity really, years ago you could park your Tiger Moth for days and the only person who would touch it was the local Policeman.