One thing that people do not realize is that tires lose pressure on there own. It is common for tires to lose about 2.5 psi per month. Tires can also lose about 2 psi for every 10 degree drop in temperature. These figures are average numbers, and your change will vary. The reason tires lose pressure with temperature has to do with the moisture in air. That is why high performance applications like racing tend to use nitrogen in their tires, because nitrogen has no moisture in it.
That being said, I find it unlikely that both rear tires have an issue, but the mechanic in me would definitely do some further investigation. I would adjust the pressure using a known good pressure gauge. I would then use a soapy water solution on the valve stem, the side walls, tread and beads. Take your time, and use lots of soapy water. Any bubbling could be a leak. If no leak is found, let the tires sit, and recheck pressure after a week to see what you have.