In some cases an item may not be there if no character has the perception to spot it, in others (such as dig sites) you can find the loot if you (as opposed to the characters) know where it is, or just by trial and error (though you'd have to know your characters were close for that to be feasible).
Apparently the spider summon has good perception, so summoning it when you think you may be near a secret can help.
