When you say 'faucet', does that mean your unit has the actual faucet ... or is it a shower hand held unit like the 2720 we have? If you meant the hand held item, could it have been that it was not held above head to drain when you drained eveything else. I experienced this recently during my familiarization process with everything. I drained using the petcocks ... and when checking a day or so later, got a face full of water from the hand held unit.
I also live in a location that can get down to minus 15-50 degrees now and then; and your comment concerns me somwhat as I was under the impression that opening the 4 drain valves, along with the interior spigots, drained the system completely ... and all that was possibly needed was a bit of the antifreeze in the traps.
Thanks
Paul