If you have air returns that are on your ceiling -- these are the vents through which hot or cold air blows out from the HVAC system -- and you see little spots that look like water stains, it's time for some investigation. Those water stains mean something is likely wrong with the system or in that area of the ceiling. While it's possible it was a one-time thing, chances are there's something chronic going on.
Humidity and Condensation
One possibility is that there was too much humidity, and the moisture in the air condensed and rolled down to the frame around the air return. When these things are on your ceiling, the frame that sits out a bit from the ceiling lets water concentrate into larger droplets (in other words, as moisture condenses, it rolls down, and those frames are kind of like the lowest point on the ceiling, so the water rolls there, and then drips down to the floor).
If there was a one-time thing where the room got way too humid (maybe you had a humidifier and set it to run too fast), then that would explain the droplets and stains. However, another possibility is that something is wrong with the condenser in your HVAC system, and it's not grabbing enough moisture out of the air before sending the air back into the room. If it's been feeling a little too humid lately, and you've had to set your air conditioner (if you've had a winter heatwave) to a lower temperature than normal, you may have a condenser problem. If you've been using the heater and haven't had to set the temperature as high as you usually would, given the temperature outside, then that's also a hint something is wrong.
One other possible problem is that you have weak spots in the ceiling around the return, and a leak somewhere else in the roof. When your roof leaks, the water gets into the crawlspace or attic, not right into the rooms in your house. The water then pools and rolls to wherever the lowest point in the crawlspace floor/your room's ceiling is. If there's a weak spot or crack there, then the water can drip through.
Your best bet is to call an HVAC or air conditioning repair company to have the HVAC system inspected. If they don't find anything, then you may have to have someone look at the roof, too. For more information, contact companies like Airwaves Heating & Air Inc.