Earl, I recently added a sign frame above my
coin boxes so I can put up a sign - and most importantly
easily change it - regularly. I haven't put one up yet about the meter running, but I do have a draft I'm planning to put in for 6 weeks or so during the spring "detailing" rush. Wording is pretty much what chaz said, "Thank you for exiting the bay when the timer is not running."
When I see people washing with the weep I always point out that "you're washing with my anti-freeze solution, thats really not a good idea." Yes I weep water to make sure my lines dont freeze - so I'm telling the truth, & semi-politely achieve my objective. No its not passive aggressive. Aggressive, maybe, passive not so much

LOL - a couple years ago I said this to a customer, his wife started hitting him and yelling the "I told you not to do that you stupid so-and-so..." Still cracks me up.
Sort of related - I have a bucket under the exhaust from my natural gas heater to catch the condensate dripping from the exhaust. A couple times a month a customer "steals" that bucket of water, dips in the FB and starts to get himself a free wash.
(For those of you who had too much to drink with dinner, the condensate is acid.)
Sometimes I write on the bucket "Acid - do not steal this to wash your car!" Sometimes I think they are getting what they deserve. Sometimes I walk into the bay and take the bucket right out from under them and tell them it is acid, go get an adult to help you wash your car.
Yeah, I know, reading that over it kinda sounds like I have bad attitude! The vast majority of the time I am actually quite good with customers,