Unexpected output on acivate plugin

When activating my plugin and only if wp debug mode is on, i get the message:

The plugin generated 255 characters of unexpected output during activation.
If you notice “headers already sent” messages, problems with syndication feeds or other issues,
try deactivating or removing this plugin.

I did not see the headers already sent message, or the other possibilities mentioned in the message.

The plugin works as expected.

I would like to read those 255 chracters to see where they come from and so be able to repair for this error message.

Where do i find the 255 characters?