I remember a story a couple of years ago regarding Chris Michalek
flying to the SPAH convention, sitting on the plane holding the
harmonica up to his mouth but blowing over the top of the holes, so
that it made no sound. After that I started using a similar
technique in situations where I didn't want to be heard playing,
except that I draw and blow VERY lightly through the harp, not hard
enough to make a sound. I found that it wasn't too hard
to "audialize" (the audio equivalent of "visualize") what notes
would be coming from the harp. It's a little tougher integrating
the overblows, but it's great for a little practice time in a
situation where it might otherwise not be possible.