A very small leak can be detected by the schnog (nose). You either have a very small leak or are still smelling the leftovers from the job you just completed. I agree with JFM626, give it a couple days.
However, if it doesn't go away on it's own then look for a small leak. Given the maintenance you just did and the fact that you aren't seeing coolant on the ground I'd guess thermostat housing. If it had a small leak it could likely only leak while running. If leaking while running it is being blown all to hell by the fan and evaporated off before it can be seen. Look for traces of dried green (or red depending on what your using) where there wasn't before. In fact, make sure your engine is nice and clean now, since you just did this maintenance and likely spilled coolant everywhere (like I always manage to do
) and then look for new traces after running her a bit.