flies/gnats in drain line

I live in North Carolina are on a septic system and there is a clean out located outside our house for the pipe between the house and the septic tank. I noticed the other day when i took the cap off several small flies emerged from the pipe. I mentioned this to a pest control guy and he said this was a good indication of a break in the line. Its a new home so I am not really sure this is correct. I have flushed the toilet on the opposite side of the home and when I checked the clean out there was no drop in water volume. There are also no signs in the yard or in the house of a leak. Do these flies only get in the line from a break or is it possible from somewhere like a vent pipe?

Check this out,
are these what your talking about

http://www.greenvalleypc.com/html/flies/drain.htm

There also should be no water in the pipe except when you flush then it should pass by the C/O and be gone.

If there is standing water and you are on septic have it pumped.

If you are on city sewer you have a blockage of some sort or the line is bellyed.

Those flies are a *****. My old apartment used to have them! Always in the bathroom---then again I discovered a 3+ year old water leak when fungus began growing. That wasn't fun to clean and repair, which I had to do myself and my landlord compensated me only $40! Bastard... Stupid PEX pipe.... Anyways... sorry....

I would think that they could go in from a vent, but you could get the line camera'd to find out.

Jason