Even if it wasn't, how long does it take for an open beer, or a soda, to go flat? That's just a release of supersaturated CO2, and from a much higher dissolved concentration than in a fermenter that has a mechanism for releasing pressure.
Maybe 2-3 days, without an airlock?
Sean, if you're decreasing the pressure in the fermenter headspace by pulling from a sample port, then it sounds like you're not using any type of liquid airlock? Or are you getting suck back (even if it is not enough to actually pull airlock liquid back into your fermenter)? And it sounds like you're bringing the temperature up for warm maturation? That would result in bubbles.
We do use a liquid airlock, a tube from the top TC port to a bucket of sani on the floor. There would have to be a pretty significant decrease in pressure for any sani to get in the fermenter.
We do bring the temperature up, on roughly the third day of fermentation. After that it stays at 72±1°F for a week or so before we cold crash it. Typically our ales will reach FG on day 5-6 (verified by daily hydrometer readings) but continue to off-gas until we crash them on about day 12.
Just my experience on a homebrew scale, I've never fermented in a container larger than 7.5 gallons where maybe things are different.
There is a really simple experiment you could do next time. A week or two after active fermentation is finished, agitate the fermenter. I'd bet money on you seeing a burst of airlock activity, indicating that the CO2 in solution hasn't come to equilibrium.