There is an excellent set of tips for clandestine meetings by an FBI veteran at Foreign Policy Magazine. You will need to log in with facebook or Google to read (or have/create another accepted account)

Spy Craft for Hacks

One glaring ‘agency friendly’ omission is, he neglects to mention if there is a cell phone along, no matter if turned off and not used, you’ve given yourself up already. Keeping this in mind, the read is quite informative-