What kind of stickers do you use on the locks? Where on the lock do you put them?

We use color coded disc locks, not stickers... Overlocks are red(keyed alike) Vacant units are yellow(keyed alike) Auction locks are Blue(keyed alike) The tenant has their own personal disc lock.

Red locks = overlocked units | Yellow locks/tags are vacants for us. I used to have a property under me that on every customers lock he would place a green round sticker and I asked him about it as it was the first time I had ever seen it done and his explanation made perfect sense but I could never get other stores on the same page. He said this way he can go down aisles and easily recognize if a lock has been changed so for instance someone has broken into a unit and placed a new lock on the unit to make it look normal so they can come back again later. Anytime he would not see a sticker he would contact the tenant to inquire if they changed locks.