Discern location should be sufficient to find the spy, unless you choose to define belonging oddly. I would only make discern location fail in one of the following situations:
1) The caster cannot describe who he is looking for in a unique manner. If you seek the writer of a letter, when the letter was actually written by two different people, you don't find both people (though I would give a hint as to why you failed).
2) The item being used is not in fact associated with the person being sought. For example, if you find someone stabbed with a knife, and use discern location with the knife to seek for the killer, this doesn't help if the knife was taken from the kitchen, rather than belonging to the killer.
Creative spies (particularly ones likely to be hunted by 15th level clerics) will find ways around this sort of problem. Generally, the tactics used to protect against scrying (don't leave anything usable as an arcane connection around) will work against discern location as well.
1) The caster cannot describe who he is looking for in a unique manner. If you seek the writer of a letter, when the letter was actually written by two different people, you don't find both people (though I would give a hint as to why you failed).
2) The item being used is not in fact associated with the person being sought. For example, if you find someone stabbed with a knife, and use discern location with the knife to seek for the killer, this doesn't help if the knife was taken from the kitchen, rather than belonging to the killer.
Creative spies (particularly ones likely to be hunted by 15th level clerics) will find ways around this sort of problem. Generally, the tactics used to protect against scrying (don't leave anything usable as an arcane connection around) will work against discern location as well.