 MISC: Rachel tells Robert Dean that when she needs to contact Brill,  she "chalks" the mailbox, then Brill crosses the chalk mark and she  makes the drop. After Dean discovers that Rachels been killed, he  uses a black, indelible ink marker to signal Brill. Brill is very  specific about his procedures, and the reason chalk is used is  because it can be easily wiped off so it will be clear when she  needs to make a new drop. If they continued to use indelible  markers, they wouldnt be able to wipe off the old marks and have  any way of knowing when a new one had been made.