Try this:

CREATE cursor TEMP (Fld1 I)

INSERT INTO TEMP VALUES (100)      
INSERT INTO TEMP VALUES (110)      
INSERT INTO TEMP VALUES (113)      
INSERT INTO TEMP VALUES (115)      
INSERT INTO TEMP VALUES (119)      

INDEX ON Fld1 TAG Temp

SEEK [100 Test TestMe]
? FOUND()              && .T. -> what!!!!!!!!!!!!!!!!!

INDEX ON STR(Fld1) TAG Temp
SEEK 100             && booom!!!!!
? FOUND() 

Maybe this is disused somewhere but I have missed that discussion.

I always use ORDER in seek, but in ONE place I didn't and viola!!!!! what I found.
When the order is based on Numeric (some kind) field you could seek strings and VFP didn't complain and above that IT FOUND() the string :-)))) (if the string begins with numbers)

But if the order is STRING based you can't search anything but strings in it.
Grrr!

SO: ALWAYS USE "ORDER" clause in SEEK :-)