The idea of an enemy sleeper agent is a central plot device in many a spy novel or movie, and certainly the idea of going to ground behind enemy lines is not unheard of in many theatres of conflict. The idea in all cases is to remain undetected until re-activated to cause harm behind enemy defences.
The trick to identifying if there are latent sleepers operating is to try and re-activate them and get them to reveal themselves. Cue any number of spy stories about false radio signals or targets to lure sleepers into the open.
Curiously, it is a similar strategy that is being used in novel treatments for infections from two disparate areas of chemotherapy, one in the treatment of HIV and the other the treatment of persistent bacterial infections.