I haven't seen it, but I mentioned the Swan that is being seen at 
Schoolhouse Pond this morning to someone who would certaily know about 
these things, and she suggested that the swan may be from a captive 
breeding/reintroduction program in Canada.  Info can be found at the 
following web site, and the Swan can be reported there as well:

http://www.wyemarsh.com/swan_sighting.html

The yellow wing tag with a 3-digit number seems to fit this program's 
customary methods, though I am not sure if others also use these 
particular tags.

Jay Keller