You are using DLP default "system" Data Identifier (e.g., the defaults that come with your install, such as for SSN or CCN detection, etc.).
You find that DLP detects test content in most cases, but you have seen that numbers prefaced or followed by the pipe character ("|") are not being picked up by your DLP detection.
Current DLP platforms, using either on-premises or Cloud Services detection servers
There is a list of "pre-validators" and "post-validators" that is part of every Data Identifier (aka "DI") by default. The pipe character or separator is not one of the validator characters in the default list.
These are not revealed in the UI and you cannot add any characters to a system Data Identifier, but the list of validator characters is available at the online help topic below (see Additional Information).
To add a character to the list of these pre- & post-validators, follow the steps below to first copy an existing system Data Identifier, thereby creating a "Custom" Data Identifier.
Note: By creating a Custom DI, you will have the option to add the pipe to the list of validators. A Custom Data Identifier only allows for one breadth of pattern , however, named "Wide" by default. To create a Custom DI that detects on a Medium or Narrow breadth - copy the appropriate pattern into your new Custom DI configuration.
See this topic for the list in 16.0.1: Configuring pre- and post-validators (broadcom.com).
There is also a Feature Request for having these validator characters exposed in the UI for system Data Identifiers: PM-3714, "Detection -- Pre- and Post-Validators for Default Data Identifiers".