20+ PD will detect Stealth +0 probably 75% of the time. 10+ PD I would say has a 50% chance.

For Stealth +25 (or more), you have virtually no chance to find them with common PD and will have to revert to using Patrollers and quite a few, to boot.

Bane Venom Charms are excellent for causing havok and psychologically. This is one great advantage that Spy Nations (Marignon, Ulm, etc) can use, by chainusing High end stealth units to drive huge amounts of death and panic on a nation.