I know it sounds crazy, but is it possible that a White Monjita would come this far north, for one thing, and at this time of year?
South of BWI, saw a snow-white bird land in a neighbor's yard (100 yards away), near the road and close to a row of evergreens. Immediately caught my eye due to the rarity of completely white birds. It was not albino, nor a gull or other shore bird.
It stayed fairly still, not moving much, hopping and turning a couple of times, for a good 60-90 seconds, then left with a low flight path.
I couldn't get a REAL close look with my small binoculars, but it clearly had no facial markings, medium-sized beak, and a very distinctive black band running the length of the wing, such that there was plenty of white above and below the stripe - just like in this picture:
http://www.oiseaux-birds.com/card-white-monjita.html
I can't find anything else that looks like it.