Dreamer
It is a mono track and the source category is entire mix. I'm confused with what you mean with output pairs? Do you mean two pairs of output drivers? (That's what I have although I'm really just using one).
 
Looks like you hit the nail on the head if it's what I described, (two pair of main outs and entire mix as source catagory).

No, if you're only using one pair then that idea is ruled out.
 
I run a headphone bus to outputs 3/4 on my interface which would add 6dB if I used "Entire Mix"

I'm not getting it. Is it just choosing "Entire"Mix" as the source category that would cause this phenomenon? I did a test, did the bounce choosing "Main Outs" as my source category. I'm still getting gain. And it's always the same increase.....
 
Total increase in gain= 3dB

Just to detail this a little more. I'm using one bus for this track, but I'm not choosing any of the bus mix enables. I am not using any Eq or automation. With or without effects I will still get an increase OF 3dB. I always leave my track volume at -0- when using an effect, and it's always panned dead center.

Check the bus track doesn't have any gain on the Trim control.

are you seeing the gain increase in the sonar meters after bounce or in another program or your audio interface's meters?  is the material you're bouncing looped audio?

Trim set to -0-. Increase is read from Sonar meters. No looped audio.

Freezing a soft synth with no addl fx added also increases by +3dB on my system.
Never have been able to figure out why...
 

This needs investigating fully.
 
I am getting NO difference when either bouncing or freezing.
 
Can you start with a blank project and insert a synth, say Dim Pro, load up a lead patch and any old midi groove
 
Play it through several times, make a note of the peak level in each case. They might vary slightly but any more than about 0.2dB would be a cause for concern
 
Then freeze the synth and measure again.
Unfreeze and bounce to track, measure again.
 
Also try the above 3 routines with the track fader set at different levels.
 

I'm just wondering if there is a track input somewhere adding to the final output.

I've lived with this from Cakewalk 2.2 on up. Kind of accepted it but always wondered if it was some dumb thing I was doing. But I could never figure it out. Updated from a Delta/Omni i/o break-out box and sound-card, to a Scarlett 2i4. Same thing. Always gain on bounce. Perhaps somebody here will figure it out.