Paid for a head gasket replacement job but now I think they may have missed something?

I have had some major repairs done to my car (2000 Honda Civic LX, 4 Cylinder). Had a few places look at it but finally sucked it up and took it to the Honda dealership.
They sent the block out to make sure they it wasn't warped, replaced the head gasket, water pump, timing belt, other belts, oil job, etc. 2 pages worth of stuff. I had the radiator and hoses and some other stuff replaced about 2 years ago. They also recommended having the catalytic converter replaced it had a "crack", but they wanted to charge $800 so I said I would have wait (Honda genuine parts are expensive!) They never said if the cover was cracked or the actual converter.
Since I got it back it's been ok but I have noticed that it is using much more oil than before. There are no oil leak spots on the carport so I'm not sure where it's going or why it's using so much.
Today, I noticed this weird spot on the carport, approximately where the exhaust comes out of the tailpipe. I think it's where I crank the car up to warm up in the morning. So the weird spot has like smut colored drops in it and so I swiped it to see what it was. It did not look, feel or smell like oil, but it kinda looked like soot when you rub it. So now I'm paranoid that the Honda place missed something major - even though they called me everyday they had it with "recommended" repairs. I hesitate to take it back because they are expensive, and the service manager treats me like I'm an idiot.
Anyone have any ideas on what it may be?

That "soot" may be where the cat converter is cracked and leaking exhaust into the passenger compartment… Would be a good idea to have a muffler shop look at the converter; you don't have to buy honda parts!

That's just condensation in the exhaust when it sit overnight, especially in cool or cold weather. The condensation mixes with soot in the exhaust and when you first start it, it will push that mixture out of wherever there's a leak in the exhaust system. Since you mentioned the converter had a crack in it, I would suspect that's where it's coming from. It's really nothing to worry about. If there were no exhaust leaks, that condensation would just come out o the tailpipe as steam. That's why you see steam coming from cars in cold weather.

Do you see blue smoke coming out of the exhaust? That's oil-burning.
(Please - don't take it to dealers - they'll replace (at high prices!) anything to get you off their backs)
Very unlikely - but they might have not torqued the bolts properly.
The convertor problem has nothing with oil-burning. There's a way, to drastically improve gas-mileage, and power. Take the convertor out - bypass it with a straight tube.
Just be sure to get your smog-certificate first.