On another question about leaving brew longer in a second or primary fermenter for a long time, many state they have done this with little to no negative affects as long as you keep your breather full of water.
I leave for months sometimes. If I suddenly need to leave and my breather is no longer active, is there a reason I couldn't pull the breather and just cork off the beer in the primary or secondary fermenter?
Like above, I've found corney kegs to be a great sealed aging container.
Couple notes.
I wouldn't allow any pressure in any glass carboy. Below is box from 6gal Italian glass carboy.
!
PSI is pounds per square inch. Conditioning and secondary can easily make 2 bars, about 27psi. Because of the surface area a carboy would fail at a fraction of that. Plastic PET fermentor should be ok, keeping a plug in may be difficult though. It can pop out under pressure or even suck into the fermentor on a temp drop.
I wouldn't use just water as an airlock fluid, use a sanitizer or vodka and keep it fresh. If the fluid becomes contaminated, along with temp drop it can bell siphon the fluid into the beer.
If it's going to be unattended for a long time I would use a tube and a 1/2 gallon sanitizer bottle. A small airlock may go dry.
As far as the condition of the beer after several months. Its best to age beer off the primary yeast if the style calls for aging. Some styles are best fresh, IPA for example. Some ales and lagers can benifiet from several months of aging. Several months in a proper fermentor will do nothing "harmful", but may not age into what is true to style.
That would be fine as long as the fermenter can handle some pressure. It might even improve your beer. But if you're traveling a relief valve is pretty much required (like a cornelius keg).
Beer that appears to not be 'active' is still generating some CO2 - before the yeast are completely dead (which will take several months) they consume stored glycogen, and the last traces of sugar in the beer. Ever open a year old home brew? Mine are almost always highly carbonated by then.
In the secondary, the pressure should not be much, may or may not more than serving pressure. Primary, however, will reach undesirable pressures very quickly. Relief valve required for sure if you have more than a few points of gravity remaining.
