That's probably what it is.
H2O is strange. It shrinks as you cool it down to +4 deg. Celsius. As you cool it below that, it actually expands as it gets colder (below +4 deg. Celsius), expands as it freezes, and then continues to expand as you cool the ice. This is why ice floats.
Also, water is virtually immune to changes in pressure. (which is why the stuff is nearly incompressible), but it actually changes it's volume pretty substantially with temperature changes. Take a look at this chart:
H2O - Thermal Expansion Coefficient
The coefficient of thermal expansion, K, changes with temperature. At 50 deg. C, or 122 deg. F, or the approximate temperature of the water in the heater when you turned it down, the thermal expansion of water is 0.00045365 per degree Celsius, or about 0.045 percent per degree C. At room temperature of about 25 deg. C or 77 deg. F, the thermal expansion of water is 0.0002569 per deg. C. or about 0.026 percent per deg. C.
So, if we take the average thermal expansion of about 0.035 percent per degree Celsius and multiply it by the temperature drop of 25 deg. C, we get 0.875 percent. Assuming a 50 gallon hot water heater, that's 0.44 gallons change in volume, or about 7 cups or 1.66 liters.
Water is nearly incompressible, so even that small change (7 cups change in volume in 50 gallons) would have resulted in a significant pressure drop IF (big if there) your water supply system was totally rigid. However, much of the volume change in the water would have been compensated for by the deflation of the water heater tank.
Still the fact that you're saying that "air" came out of your faucets means that the pressure drop was sufficient that dissolved oxygen came out of solution from the water in your hot water tank and your water supply lines. Air is 78 percent nitrogen gas, but the 21 percent that's oxygen is much more soluble in water than nitrogen. So, when you boil water, the bubbles that form on the bottom and sides of the pot are almost pure O2. So, most likely, the stuff that came out of your faucets wasn't "air", it was nearly pure O2.
I think you're right that what happened was that the hot water in the water heater cooled down during your vacation, and the resulting change in the water pressure allowed dissolved oxygen to come out of solution and form bubbles in your supply lines.