Pull your phone out and pretend to be texting someone while you jot down some notes and make some observations. Or put the phone up to your ear and pretend to be talking on the phone. Assuming, of course, that you haven't been warned about not using your phone on the shop. People in airports are glued to their phones, so standing around for a few minutes while you "take a call" or "send an email" won't look unusual at all.
Shopper in California's Bay Area