Security Level Tau appears to be a security procedure executed by the Space Pirates in Metroid Prime, possibly when Samus Aran invaded their Phazon Mines. It seems to be that if in effect, all personnel are needed, and so all leaves are cancelled.
"Security Level Tau will be in effect until further notice. All leaves cancelled pending downgrade of alert status."
- Tau is a letter of the Greek alphabet equivalent to "T".