This leads me to think the thefts, if that is what is happening, are an inside job...
She shrugged. "That's hard to say. This site has turned out to be an archaeologist's dream. The squares that were disturbed were turning up a lot of artifacts, so chances are something was found. There's no way of knowing for sure, though."
It is past time for light to shine on these folks; a series of discreetly placed battery-operated trail cams would be beneficial. They are easy to place, work only when triggered, and are easily camouflaged....