What is that ASCII Art From the Portal Credits
According to the ASCII art it's "GLADOS"
Check this page. Look at the second post, click Show and scroll down to GLADOS you'll see the same ASCII art there in reference to that name.
EDIT: In case that page goes away here it is
"GLADOS1" "[12.000] #+ @ # # M#@ "
"GLADOS2" "[12.000] . .X X.%##@;# # +@#######X. @#% "
"GLADOS3" "[12.000] ,==. ,######M+ -#####%M####M- #"
"GLADOS4" "[12.000] :H##M%:=##+ .M##M,;#####/+#######% ,M#"
"GLADOS5" "[12.000] .M########= =@#@.=#####M=M#######= X#"
"GLADOS6" "[12.000] :@@MMM##M. -##M.,#######M#######. = M"
"GLADOS7" "[12.000] @##..###:. .H####. @@ X,"
"GLADOS8" "[12.000] ############: ###,/####; /##= @#. M "
"GLADOS9" "[12.000] ,M## ;##,@#M;/M#M @# X#% X# "
"GLADOS10" "[12.000].%= ######M## ##.M#: ./#M ,M #M ,#$ "
"GLADOS11" "[12.000]##/ $## #+;#: #### ;#/ M M- @# :"
"GLADOS12" "[12.000]#+ #M@MM###M-;M #:$#-##$H# .#X @ + $#. #"
"GLADOS13" "[12.000] ######/.: #%=# M#:MM./#.-# @#: H#"
"GLADOS14" "[12.000]+,.= @###: /@ %#,@ ##@X #,-#@.##% .@#"
"GLADOS15" "[12.000]#####+;/##/ @## @#,+ /#M . X, "
"GLADOS16" "[12.000] ;###M#@ M###H .#M- ,##M ;@@; ###"
"GLADOS17" "[12.000] .M#M##H ;####X ,@#######M/ -M###$ -H"
"GLADOS18" "[12.000] .M###% X####H .@@MM@; ;@#M@ "
"GLADOS19" "[12.000] H#M /@####/ ,++. / ==-, "
"GLADOS20" "[12.000] ,=/:, .+X@MMH@#H #####$="
I agree with badp, I think it is one of the security cameras found throughout the levels.