Air in line question

[Ludington]

I just find this odd, when we go on vacation, I shut the main water valve off to the house, turn the water heater off, but leave the well on. I can monitor if the well ever turns on, and it doesn't, it never drops a pound. But on return and turning the water to the house back on, I get air comming out of every faucet, toilet, bathtub. Cold and hot water. You would think there was a leak , but all plumbing is exposed and not a drop anywhere. And never a drip or air in the lines once repressurized. Just wondering where these air pockets are comming from. Any ideas ? It doesn't seem to be a problem, just an odd occurrence that I only see if I'm away for a while and have turned the water off.

 

[Ludington]

Solved it myself, I had forgotten I have a water filter ( Gac/kdf) that backwash every 6 days. You only need to change the media every 7 years, so I forget it's even there. But with the water turned off and that valve opens, I'm sure the house water from the the floor above it just drains out. I'll turn the thing off when we go this time and see what happens, but I'm sure that's it.