I was recently greeted by this icon on my dashboard, so I got my gauge and checked all the tires, each of which was at about 25 psi instead of the recommended 30. I've not had any service for the vehicle and have only visited a gas station about 4-5 times this year, so I used this occasion to drive over to my local Subaru dealership, which offers an "express" service for minor problems.
While the technician was checking and filling my tires, I mentioned that I could not see any evidence of deflation and assumed that this was just a result of neglect and cold weather. Then I learned something. He told me to never judge the inflation of a tire by visual examination. He has seen tires with pressures of 10-15 psi that were still upright and visually "ok" - because the rigid sidewalls of modern tires keep them upright even when underinflated. You learn something every day.
I was in and out in 15 minutes and there was no charge. That's good service. And good advice to check tire pressures routinely during fillups or before trips.