I don't believe leaving a plane open is a good idea - unless it is a dead simple worthless spamcan with nothing in the panel.
There is a lot of opportunists around, who nick anything simply because they can do so easily.
People nick stuff out of open hangars as they walk past. And it takes only seconds with an allen key of the right size to remove a £10,000 avionics item, which you can sell on U.S. Ebay.
You've got to keep the plane locked, not leave any obvious stuff in there like a handheld GPS or headsets, and if possible keep a cover over the cockpit so that passers-by (there is never a shortage of candidates for casing the joint at any GA airport; most are plane spotters but you can never be quite certain) don't get an opportunity to work out, perhaps over a period of time, what is in there.
Thankfully this problem is rare, especially at airports with "security".