More than likely, the evaporator under the dash in the heater box is leaking. Its a real common problem, and usually when you have the box out, the smart thing to do is do the evap and heater cores at the same time. Its about a 5-10 hour job depending on your experience to swing the dash out of the way to get the box out.

For the moment, try one of the cans as suggested above and see if it fixes it. If you can go all summer and not have to worry about it, then its a small leak. A/C systems are like brake systems, they either leak or they don't. Also, make sure they can you add into the system has dye in it, this will help a technician find the leak later on if you choose to have someone fix it for you---or you can buy a UV light to see yourself where the leak is--unless its in the heater box. No way to see that until its apart.

Also, check the condensor up front to see if it has any staining on it running down it. This would be the oil in the A/C system dripping out. Not uncommon to have road debris puncture it--even slightly.

If its leaking from a hose, sometimes that is harder to find, the dye can help here, and again, look for accumilating dirt clinging to the oil leaking out. There is no warning lights to indicate low A/C refrigerent, a low pressure switch shuts off the compressor if there isn't enough refrigerent and a high pressure switch shuts it down to keep the sytem from self destructing with to much pressure.

Hope that helps some.