[masraum]

Sounds like pretty much every other car/tire out there, they lose air over time, sometimes at different rates.
Get an air gauge and a compressor. Check them before they get that low and then put air in them.

My '08 Boxster and my wife's '18 Outback will show you on the dash the amount of air in all 4 of the tires. This set of tires on the Boxster is weirdly stable, but that's not usually the case. This set of tires on my wife's car is pretty good, but the right rear tire loses air faster than the rest. Over 3-4 months, she may lose 1-2psi in 3 of the tires and the 4th may lose 4-5psi in that same time. I try to top it off before it gets that far along to keep them close, and will often add an extra 1-2 psi to give myself a little more time.