Unexpected output alert

We're seeing an increase in the following error message appearing on the CMS Builder backend on our in-house dev server.

Unexpected output was sent by a program library or plugin.
Output was "

" (4 chars)
Developers: Check plugins and library files for accidentally added whitespace or other characters.

The character count varies The sites that are now throwing this do have additional plug-ins in place but the exact ones vary a bit. Before I run though a range of experiments, I thought I'd ask the experts about this.

Any idea what I should be looking for that might cause this?

Many thanks,

Jayme