A leaking evaporator is the likely suspect for Freon loss if there's no visible oily residue elsewhere that points to a leak. But, the only way to confirm it is with dye and a black light or to use a Freon sniffer. The good news is that the 3rd gen AC evaporator is easy to change and the part isn't terribly expensive. The hardest part is finding someone to evacuate the system and recharge it. On most vehicles including most 4Runners the dash has to come out to get to the evaporator. But, on the 3rd gen it's accessible after removing the glove compartment.