Can't Exchange "Dirty" Money.

Changed 10 x Australian $100 notes the other week at an exchange booth in Chiang Mai. Was handed one back and told no good because it has a little writing on it. Took it home and with a little help from some isopropyl alcohol and a cotton ball the writing came off. Yesterday I changed some more at a different exchange shop. Checked every note before I left and no writing. As she was checking them she showed one to her co-worker and spoke in Thai. I asked anything wrong. She showed me a small black smudge and said dirty. I said seriously how can you have perfectly clean money in circulation and that many Thai notes I get from shops, banks and ATMs have writing on them. She just exchanged it and said no more. I went straight to a bank and deposited via cash deposit machine. As I did I had a quick look at a few of the notes and at least 5 had writing on them.

I guess a bit of money laundering is needed before my next exchange.