Why does AntlrWorks 2 display warning 125 (implicit definition of token in parser) in this case? - antlr4

Question

I have a separate lexer and parser grammar (derived from the sample ModeTagsLexer/ModeTagsParser) and get a warning in AntlrWorks 2 that I don't understand:
warning(125): implicit definition of token OPEN in parser
If I replace the OPEN rule with '<' the warning goes away. I wonder what the difference between OPEN and CLOSE ist which get's no warning.
I'm using antlr-4.1-complete.jar and 2013-01-22-antlrworks-2.0.
Lexer STLexer.g4:
lexer grammar STLexer;
// Default mode rules (the SEA)
OPEN : '<' -> pushMode(ISLAND) ; // switch to ISLAND mode
TEXT : ~'<'+ ; // clump all text together
mode ISLAND;
CLOSE : '>' -> popMode ; // back to SEA mode
SLASH : '/' ;
ID : [a-zA-Z0-9"=]+ ; // match/send ID in tag to parser
WS : [ \t]+ -> channel(HIDDEN);
Parser STParser.g4:
parser grammar STParser;
options { tokenVocab=STLexer; } // use tokens from STLexer.g4
unit: (tag | TEXT)* ;
tag : OPEN ID+ CLOSE
| OPEN SLASH ID+ CLOSE
;
It even persists if I rename the rule slightly and remove the additional mode:
lexer grammar STLexer;
Lexer (modified):
// Default mode rules (the SEA)
OPPEN : '<' ;// -> pushMode(ISLAND) ; // switch to ISLAND mode
TEXT : ~'<'+ ; // clump all text together
//mode ISLAND;
CLOSE : '>' ; // -> popMode ; // back to SEA mode
SLASH : '/' ;
ID : [a-zA-Z0-9"=]+ ; // match/send ID in tag to parser
WS : [ \t]+ -> channel(HIDDEN);
Parser (modified):
parser grammar STParser;
options { tokenVocab=STLexer; } // use tokens from STLexer.g4
unit: (tag | TEXT)* ;
tag : ID OPPEN ID+ CLOSE
| ID OPPEN SLASH ID+ CLOSE
;