Secondary in keg

I secondary/lager my lager beers in a keg with a 1/2" shorter dip tube than a normal keg. My poppets are a little screwey, so I connect gas line and beverage line just to hold pressure. I do not keep the keg under forced CO2 pressure, as I'm not looking to carbonate the batch until I transfer it to a serving keg.

My question is this. I've noticed that over time the keg loses pressure. If I pressurize the keg to purge air and seal the lid, then cut the gas, there will be no audible gas release when I open the relief valve. My thought is that the marginal amount of CO2 in the keg is disolving harmlessly into the beer leaving no pressure. The question is: Is this normal?

I'm not too worried about air contamination, because I'm not seeing any way for air introduction. I just find it weird that a pressurized keg will become unpressurized in 24 hours.

Yes.

Correct. I leave my conditioning kegs next to the keg fridge for the first few days. The first day or two I'll top it off with pressure 2 times a day. Every few hours the keg can soak up more CO2. After it stops soaking up more I'll leave for another day, pressurize once more if at all possible, then store. Check it every now and then for a good seal. This of course can be avoided by putting priming solution (as you would for bottling) into the keg and letting nature run it's course. If there aren't any leaks it won't break a seal and will be carbonated when you go to tap it.

EDIT: Just saw this, "as I'm not looking to carbonate the batch until I transfer it to a serving keg." By filling the head space with CO2, it absorbing into the beer, refilling the head space, you are going to be "carbonating" anyways. I'd just as soon remove the pressure relief or one of the posts and install an airlock.