Per CIS hardening standard: "'+' entries in various files used to be markers for systems to insert data from NIS maps at a certain point in a system configuration file. These entries are no longer required on Solaris systems, but may exist in files that have been imported from other platforms. These entries may provide an avenue for attackers to gain privileged access on the system, and should be deleted if they exist."
You may also want to check other files: grep '^+:' /etc/passwd /etc/shadow /etc/group
