RMAN-06054: media recovery requesting unknown archived log, Where is SCN kept?
Each redo log file (and archived redo log file) contains starting SCN and ending SCN. In case it is a last redo, the ending SCN is 0xffffffffffff.

nap01:~/oradata/jt10g$ strings redo01.log|head -3
z{|}
JT10G
Thread 0001, Seq# 0000000004, SCN 0x0000000b05b5-0x0000000bd34f

nap01:~/oradata/jt10g$ strings redo02.log|head -3
z{|}
JT10G
Thread 0001, Seq# 0000000005, SCN 0x0000000bd34f-0x0000000bf612

nap01:~/oradata/jt10g$ strings redo03.log|head -3
z{|}
JT10G
Thread 0001, Seq# 0000000006, SCN 0x0000000bf612-0xffffffffffff


Until database sees 0xffffffffffff it keeps asking for further logs. But it is not a problem, you can specify UNTIL SCN or UNTIL CANCEL for your recovery (this means you don't want a complete recovery, in other words you want to lose some recent data).