When a latch inference is discovered during HDL synthesis, XST reports the following HDL Advisor message:
"WARNING:Xst:736 - Found n-bit latch for signal <$n00xx> created at line xx."
The listing for "n" is the width of the latch. If latch inference is intended, you can safely ignore this message.
Some inefficient coding styles can lead to accidental latch inference. (Xilinx Answer 13979) lists a number of ways to avoid latch creation.
This particular message (#736 versus #737) is given in this case because XST inferred the latch on a line of HDL code that contained multiple macro inferences. For example, the latch in question might be combined with a mux or a counter, so the output of the latch might not be explicitly maintained to be reported to you.