I am running Hudson 1.285 on XP SP2. From time to time I get builds triggered by
no changes. The build status page says:

Revisions

• svn://myserver/trunk/path1 : 19463
• svn://myserver/trunk/path2 : 28221
• svn://myserver/trunk/path3 : 27133
  No changes.
  A SCM change trigger started this job

And here's also the Console Output:
started
Updating svn://myserver/trunk/path2
At revision 28221
Updating svn://myserver/trunk/path3
At revision 28221
Updating svn://myserver/trunk/path1
At revision 28221
no change for svn://myserver/trunk/path2 since the previous build
no change for svn://myserver/trunk/path3 since the previous build
no change for svn://myserver/trunk/path1 since the previous build
[MyApp] $ cmd.exe /C '"ant.bat -file build-hudson.xml DoAll && exit %%ERRORLEVEL%%"'
Buildfile: build-hudson.xml
..

So a build is triggered even though apparently no change was detected.
It should report the cause which triggered the build.