One time I remember I got called back was after a customer had a break-in and the CID "dusted" her window.
The "dirt" left on the glass was finger print dust.
Funny stuff, that. It won't rub off, wash off or scrape off. I tried everything and came to the conclusion it must be on the inside. (at that stage I didn't know what it was)
The custy wasn't in so I left a note saying the dirt was on the inside so I couldn't get it off.
Got an irate phone call that evening saying it was definitely on the outside, so I went back (again!) to see. It still seemed to me it was on the inside, so I went inside and tried getting it off. It was outside!!
Custy was getting a bit sarky by now so I gave her my scrim and told her to have a go.
She had to agree it wouldn't come off.
I told her to get in touch with the police, but she never did. I did the job for another few months till she moved, and the stuff was still there

Cheers,
Ian