Pour some water into the pan and verify that it drains properly. You may have had a temporary blockage which resolved itself. More likely the pan may not be properly pitched towards the drain line. Water may have accumulated in the past and then evaporated. A long period of humid weather may have created more condensate then the pan could catch and evaporate. Check the drain line itself that it has a continuous downward pitch.

The most likly culprit is alge and dirt restricting drain line. Even trained techs can't rule out line restriction by visual inspection alone. It is customary or should I say should be, to clean the line when the tech does annual maintnance. It sounds like you are willing to diy maintainance and I for one believe it's possiable for the owner to do a better job than they will recieve from many contractors. So here you go.
First off,it's likly the origional drain line is still in place from the closet to sewer connection. Pressurizing the line to force debris out can be done with a"FAT"tire valve stem with the schreader removed and portable air tank. Adding water and blowing out line repeated 2 or 3 times can be benificial. Putting an algaside in the pan/drain quarterly is a good preventive measure. The common home remedy is 1 oz laundry bleach diluted in 6 oz water. A word of advice. If cooling is not used for 30+ consectve days,discontinue bleach at least a month prior to discontinued use and don't treat with bleach during periods of intermentant use. Bleach is mildly corosive if alowed to remain in the pan for long intervals. Condensate will flush out the pan while in use. There are tablets specifily for hvac condensate use but some of those are more corosive than bleach so use what you may. Unless you operate a woodworking shop inside your condo,30 day filter changes is overkill. If you are useing 99cent filters,they are better than no filter at all but just barly. At minum,use the cheapest pleated filter(about $3 ea) you can find and above all,make certain it fills the rack alowing no air to bypass filter. If you have respitory or sinus problems,step up to a $8 to $15 micro filter. You will not harm the unit by changing filters every month but one good filter less often is far better than half dozen el-cheapos over the same period. There is not a majic interval. A good house keeper that doesn't have pets or younguns bringing in dust might be fine with 6 month intervals. Your filter will have a destint grey fuzzy coating if retained for maximum time.
That should go a long way on preventing condensate drain problems and afford some bonus rewards in the process.

mike ..thanks for your reply.
My carrier blower unit is designed / set up whereby a drain pipe leaves the case right below the coils and the plastic pipes lead to a drain. The blower unit sits in a pan which captures condensate if the drain is clogged. If there is too much condensate that ends up in the pan there is an emergency float switch that shuts the ac off to prevent a flood. See the attached picture. I assume that there was a temporary clog as you suggested which was the problem Thanks for your help!

I am confused. You say you have a Carrier air handler but you posted a picture of a Trane unit. It looks like you have a clear trap with clean out plugs. This is good so you can visually see the condensate and clean it out. The traps usually come with a clean out brush. I see wires leading to the trap. Is the trap also an overflow switch?

Where does the PVC pipe go after the trap? Does it lead to a drain or a condensate pump?

I disagree about bleach being mildly corrosive. It is quite corrosive. I would not use it on metal without extensive flushing immediately afterwards.

Thanks for your replies! My mistake - it is a Trane system. The pvc drain pipe is routed to a floor drain . The closet was built to house the air handler.

I assume the full bottom drain pan was due to a freak clog that was cleared when I used the brush in the clear U trap. The trap includes an "EZ trap" cutoff device which shuts off the ac if the drain pipe clogs leading to a higher than normal water level in the U trap

If the outlet end of the condensate drain is accessible, using a wet-dry shop vacuum on the outlet may be a very effective and quick way to clear out the drain line.

It is nice that you have the EZ trap cutoff device. The installation location is not the best. If the PVC pipe between the coil and the cutoff device is clogged, the cutoff will not be able to detect it. The shut off device should have been installed at the output of the coil. My Carrier coil has two drain connections, one higher than the other, where the cutoff is installed.

It is also a strange you would be a clog after on a new system only nine months of use. Check the drain often to avoid future problems.

There is a terrific little device called a Gallo Gun. It is a CO2 cartridge with a hose with a rubber cone on it. Plug the vent on the drain, put the cone against the condensate pan opening and pull the trigger on the GG. A burst of hi pressure compressed air will blow out the plug...hopefully.