HI I just changed my own oil on the es350 last week and if it is oil leaking from your car I can make a couple of guesses that you dealer might have messed up on.
1) could be the drain bolt gasket like you said but if he did reuse the old gasket that is despicable and if he did not, he might have gorilla tightened it.
2) If not 1) then the oil filter has 2 rubber gaskets inside of it. It is possible they forgot one or dislodged one when putting them back and is causing this leak.
3) There is a cap which they might not have tightened. You can actually tighten this yourself if you have a 3/8in ratchet without even lifting the car. Maybe they forgot to put this on!?!?
4) of course the last thing is did they tighten the filter housing itself. You can actually tighten this by hand but the space is a bit tight with lifting the front.
BTW unlike regular filters, there really isn't a whole lot to tighten. You screw the cap in until it bottoms out and the whole filter can even be tightened by hand as mentioned above. 15 minutes is too suspiciously fast to do the whole oil change because I even let the oil drain for a good 5-10 minutes.
You should look under your car and see if you see oil from the drain bolt or the filter housing. You don't even have to lift the car. Just get a flash light and get down and look. Don't know what it looks like? Follow this link: http://members.cox.net/n0v8or/avalon_oil_change.pdf
This pdf is for an avalon but they have the same oil filter. It is located kinda near the passenger side wheel.
Also, I remember when I got my 5000 complimentary oil change, I only got 5.2 oils put in. that's what it said on my invoice. I wasn't educated at the time but now I suspect they maybe didn't even change my oil filter! I had a toyota dealership do my next two oil changes and I just started doing my own last week.
Regardless, I would be professionally angry about it and maybe mention that when you get the survey afterwards, that how they respond to this incident might determine how you rate the service you were provided.