Likely a non-issue, but....

[DariaVader]

Now you have my curiosity up......because I've done exactly what you do and I mark them on the edge. However, the mark always seems to wash off when I clean the pillows in the morning

I mark them on the middle of the side that doesnt show the size, and a black sharpie holds the best. Dont get them wet right away after. I make a sizable mark.... it fades, but not on first wash (or second).

[Mark Douglas]

Knowing what I know now if I have reason to believe I need a test but the establishment does not I would go to any number of places that offer reasonable prices for private pay testing packages. Reasonable compared to what they bill insurance providers.
I'm not a doctor nor do I play one on TV but I can read and reason and make use of online screening tools.
Searching thyroid testing will lead to the info needed.

[TyroneShoes]

...agree with TyroneShoes... I noticed when my P10 mask reached the 3 month mark, my AHI started to creep up. Also noticed at that point, the amount of air coming from the exhaust is less. I have no scientific numbers to support this, only by holding my hand in from of exhaust and noticing the difference....

Due to these sorts of concerns, I performed a little test. I took the brand-new P10 I received, prior to use, and blocked the port and tried to blow through it. Then I repeated it with the 3-month old version. The resistance to flow was identical.

As far as a film accumulating on the mesh, what ever could that be? 70% of what goes through is humidified air, essentially air and water molecules. The rest is exhaled air, which is CO2 and water molecules. The mesh is tiny, and on purpose to create Venturi back pressure on the exhaust that changes inversely with pressure (high pressures do not need as much venting because the air is venting faster due to the higher pressure, so the increased Venturi effect modulates this as pressure rises allowing equivalent venting for different pressures through the same mesh) but there is nothing that could accumulate from usage, it appears.

[DariaVader]

soooo.... 2 nights back in the very low AHI but restless too and tired the next day, and last night another > 1 and feel more rested. Think I am going with the "Finally getting deeper more restful sleep after 5 months continuous CPAP use" diagnosis

[retired_guy]

Now you have my curiosity up......because I've done exactly what you do and I mark them on the edge. However, the mark always seems to wash off when I clean the pillows in the morning

I mark them on the middle of the side that doesnt show the size, and a black sharpie holds the best. Dont get them wet right away after. I make a sizable mark.... it fades, but not on first wash (or second).

Of course Daria does have that little black spot on the tip of her nose now. But we wouldn't want to say anything.........

[worn_out_in_lebanon]

As far as a film accumulating on the mesh, what ever could that be? 70% of what goes through is humidified air, essentially air and water molecules. The rest is exhaled air, which is CO2 and water molecules.

According to Wikipedia (for what that's worth)
Breath gas analysis
The area of modern breath testing commenced in 1971, when Nobel Prize winner Linus Pauling demonstrated that human breath is a complex gas, containing more than 200 different volatile organic compounds.

Maybe some of that stuff (200 voc's) can deposit on the vent mesh over a period of time. Also I have a confession to make - I touch my vent sometimes... Skin oil could start to accumulate over time from touching the vents, like when I'm testing if air is coming out. Seems like when I wash the mask (weekly) vs the nasal pillows (daily), the vents flow better. This is based on a couple of washes so far.

Nothing scientific, just my perception.

That reminds me... I bought a Boehringer Inspiratory Force Meter on ebay a couple of weeks ago. (so I could measure actual pressure at the mask on my set up - but haven't gotten around to it yet) I ought to rig it up to check the "back pressure" on a freshly washed vent compared to a week old unwashed vent and see if there is a measurable difference, by blocking the tube and exhaling through it.

[worn_out_in_lebanon]

Think I am going with the "Finally getting deeper more restful sleep after 5 months continuous CPAP use" diagnosis

Very glad to hear that.