lastleg and Mental Granny had some excellent tips, that I'll add to. When you search, concentrate on one building at a time and COMPLETELY search it. In the barns, after you've checked the walls and hidey holes, go over the TOPS of all beams and rafters. There could be holes drilled into the top surface to drop in coins or rolls of bills. To add back strength, they could hammer a dowel back into the hole on top of the goodies. While checking the beams and rafters, look for tacks or nails that are out of place; such as a nail sticking out a little bit from the wood's surface. Consider tying a long string with a weight on one end (a plumb bob or nut or even a rock) and let it hang down to the ground. The rest you can figure out.
If there are indications of an old chicken coupe, check it out THOUROUGHLY. Chickens and Ghenny Hens (yes, that's misspelled) were the best burglar alarms in existence. And their daily scratching around looking for bugs and sand to eat would quickly disguise any digging by the owner. (For you city slickers, chickens swallow sand particles for their gizzards; figure the rest for yourselves.)
Find the outhouse location and search it GOOD. If you can get permission; dig out the pit for bottles, coins, etc. Use a sifter to get it all.
If there's any old farm equipment in the barns, check it for hidden caches behind panels, inside gearboxes, etc. You can check these places without destroying them. Simply pull off access panels and replace them afterward. DO NOT LEAVE THEM DISMANDLED.
If any large, old trees are in the yard, use your detector to check the line between the house corners and the tree. If there is an orchard close by, check between the trees, in line with eachother.
Thouroughly check old flower beds with your detector.
Be very careful of old wells and cisterns. They could cave in with you. Old cisterns, sometimes, hold old household trash.
Speaking of household trash; look for the area where household trash was dumped.
Books, books, and more books have been written on this subject. The best one is
Search, by Warnke.
Good luck.
