Yes, kegs with liquid in them will absorb the gas. That's why your beer carbonates, it absorbs the gas.
Yeah, I get that ... but only one keg seems to continually require gas after sitting w/the gas off for a few days. It has been under gas since mid-November, surely enough time to reach equilibrium. The other two kegs do not take on gas in the same way. The suspect keg, exhibits the same behavior with all three of the CO2 lines, taking on gas. There must be a leak somewhere but I'll be damned if I can find it.
The posts are tight, greased, and not visibly leaking.