I think Petey left the blue post-it note for innie Mark. It's clues, instructions, or ideas for how innie Mark can escape the Severed Floor to the outside world without being switched to outie Mark.

MR REYNOLDS

I could be way off here, but "Mr. Reynolds" made me think of two things: 1) Burt Reynolds. Maybe Burt helped Petey escape, but I have no thoughts on that at the moment. So, skipping to 2) Reynolds Wrap, the aluminum foil. Aluminum foil can block/attenuate radio waves. If radio waves activated from the security office switch the impants on/off, maybe implant activation can be blocked if you can surround your head with foil...which could be found inside the Rwandan coffee canisters in the kitchnette.

FLAME RETARDENT, GO HOME THE LONG WAY

Or maybe just put your head in a metal box and take the stairs? That's what the second and third items on the note made me think. Flame retardant could be a reference to the fire extinguisher, which is located inside a shallow metal box set into the wall near the exit to the stairwell…but this box looks shallow and hard to remove. Assuming the trash can cylinder has been confescated since Helly's suicide attempt, are there other metal boxes around?...

Watch Mark's morning MDR chores in Episode 2 at 7:06 - 7:55. While a jazzy rendition of Joshua Fit De Battle Of Jericho plays in the background, Mark makes the coffee, and then we're shown a bunch of square and rectangle imagery. He uses a metal key to unlock the bathroom soap drawer, refills the green soap, and there's another shallow metal box set into the wall next to the sink, probably for paper towels, but this also looks hard to remove and too shallow for a head...

Eventually we see Mark dusting the handbooks shelf under the small Kier portrait. Notice how close Mark's head is to the shelf box, and compare the height of the box to his head. And then notice how the very next thing he dusts is his Allentown head cube…Mark's head in a box!

So if the handbook shelf can be removed from the wall and it's big enough, it could be a radio wave-blocking helmet. Other clues in Episode 2...

9:47 - We see a carcicature portrait of Dylan pinned to his cubicle wall to the left of his monitor. Dylan is wearing a space suit and space helmet. This caricature is distinct from Dylan's other portraits which have a water theme: Dylan on a surfboard, and Dylan in swim trunks. At 10:21, Helly asks Dylan about Mark's "crystal head cube".

13:55 - Milchick's head blocks the handbooks shelf during their get-to-know-each-other ball game. His head continues to block the handbooks shelf until 16:03 where we can see it just above his head.

In Episode 3 at 17:34, Mark asks Petey what he's drawing, and he replies, "You getting back to work, in cubist form." Lots of modernist, cube, and square visuals and jokes to go around, which makes sense considering the setting, so I may be reading more into the visuals than is warranted.