Signs for when we have visitors - mainly the IL's

[Mrs P]

This is tricky. I guess it depends on your relationship with them and whether you think they would take the notes in the spirit they are intended, or if they would be mortally offended by it?
Perhaps you could make it more like these are notes you've put up to remind yourselves (as well as any visitors) so that they don't feel overly targeted by it?
Edit to add could you use one of those toilet seat locks? And a little sign saying please flush after every use?

[Floweryfields]

I would feel really offended if my family started putting up reminder cards for me, sorry, but I wouldn't.