[aristeides]

I'd guess they're using two-way radio units.

If they are, their communications can be intercepted. NSA, are you listening?

[Lessismore]

NSA, are you listening?

They're listening, but they never say anything.

[Not_Who_U_Think]

If they are two suspects, they wouldn't use an open comm link. Use a couple of Nextel phones or any with the 'chat' function and you're both anonymous and encrypted.